The Sky This Week , March 17 – 24, 2020 !

The Sky This Week, 2020 March 17 – 24

Spring has sprung!
Orion, imaged 2011 December 31 from Morattico, Virginia.
Orion Orion, imaged 2011 December 31 from Morattico, Virginia
with a Canon EOS Rebel T2i DSLR

The Moon graces the pre-dawn sky this week, scudding along the southern reaches of the ecliptic as she wanes through her crescent phases.  New Moon occurs on the 24t at 5:28 pm Eastern Daylight Time.  If you get up early in the morning of the 18th, find a spot with a good view to the southeast and you will be rewarded with a beautiful grouping of the Moon, Mars, and Jupiter forming aa tight triangle just over two degrees across.  On the following morning Luna may be found several degrees to the southeast of Saturn.

The Globe at Night citizen-science program campaign for March is underway and running through the coming week.  The featured constellation is Orion, which should be visible to almost anyone under any sky.  Think of this as an opportunity to introduce your home-bound kids to astronomy and the concept of scientific observation.  The procedure is simple.  Locate Orion in the sky; he is prominently placed just west of the meridian at the end of evening twilight.  Open the Globe at Night web app and compare your view with the various star charts on the page, then fill out the observation form.  Your observations will be compared to those compiled in 2011 and 2012 to determine the effects of new LED street lighting programs that are replacing older technologies.  You are encouraged to make multiple observations since local weather conditions may impact your nightly views.  I also encourage all to support the program throughout the year.  “Sky awareness” is the first step to the rewarding experience of amateur astronomy.

The vernal equinox occurs on the 19th at 11:50 pm EDT.  This moment marks the instant when the ecliptic longitude of the Sun reaches zero degrees and begins another circuit around the sky.  This is the earliest occurrence of the equinox since the year 1896, and we will continue to see equinoxes on the 19th (in EDT) every four years through the year 2096.  We then have a 40 year period before the equinox once again starts a four-year cycle of falling on the 19th.  So why does this happen at all?  We can blame the calendar and the leap-year cycle.  The explanation is rather lengthy, but suffice it to say that or Gregorian Calendar system will keep the equinox occurring close to the astronomical equinox for quite a while to come.  We probably won’t accumulate a full day’s error until somewhere around the year 5000!

Many people think that the date of the equinox is when we also have equal lengths of day and night, but this is not the case.  On the 19th the length of daylight is four minutes longer than that of night.  This is due to the effects of atmospheric refraction and the apparent diameter of the Sun’s disc.  Sunrise occurs when the first visible portion of Old Sol’s limb crests the horizon in the morning and sunset occurs when the last sliver disappears at sunset.  By these criteria the actual day of “equilux” occurred on March 16th.  From now until the next “equilux” day on September 25th we will have more daylight than darkness.

Bright Venus continues to dazzle in the early evening hours.  She is rapidly moving northeastward along the ecliptic and now stands prominently above the western horizon.  As bright as she is, she won’t reach her peak brilliance until late April, but if you find yourself in a dark-sky location try to look for shadows cast by our sister planet.  Use a white shirt or a piece of paper for this interesting experiment.

Mars, Jupiter, and Saturn may be found low in the southeastern sky before dawn.  Mars is the faintest of the three, but you should easily be able to identify him by his reddish tint and rapid motion.  On the 18th the waning crescent Moon passes just to the south of Mars and Jupiter, and on the morning of the 20th Mars passes just under a degree south of Old Jove.  Mars then draws a bead on Saturn, and the red planet will pass Saturn on the morning of the 31st.

The Sky This Week, February 25 – March 3, 2020 !

The Sky This Week, 2020 February 25 – March 3

Taking a leap (year).
Crescent Moon and Venus, 2018 April 17, imaged by Dr. Marc Murison at USNO's Flagstaff (AZ) station.
Crescent Moon and Venus, 2018 April 17
Imaged by Dr. Marc Murison at USNO’s Flagstaff (AZ) station
with a Canon PowerShot G3 X.

The Moon waxes in the evening sky this week, climbing northward along the ecliptic toward the bright stars of the Great Winter Circle.  First Quarter occurs on March 2nd at 2:57 pm Eastern Standard Time.  Luna starts the week near the dazzling glow of Venus, passing closest to the planet on the evening of the 27th.

2020 is one of the years in our calendar in which February has 29 days.  We call these events “Leap Years”, and most people think that this is an event that happens every four years.  We need to make this periodic adjustment due to the fact that the time it takes the earth to complete one orbit around the Sun isn’t exactly 365 days.  The true duration of the so-called Tropical Year is 365.2422 days, so to keep the calendar in synch with the astronomical seasons an extra day is periodically required.  The origins of this practice dates back to the rule of Julius Caesar, when the four-year leap year cycle was introduced in his calendar reform in the year 46 BCE.  While the Julian Calendar gave a fair approximation of the true year, it was 11 minutes longer than the tropical year, resulting in an accumulated error of one day after 128 years.  By the 16th Century CE this error amounted to 10 days, wreaking havoc in religious calendars that fixed feast dates by the occurrence of the vernal equinox.  A number of calendar reform schemes were proposed during the late 15th and 16th Centuries, but it was ultimately scholars working for Pope Gregory XIII that came up with the system that we still use today.  In 1582 he promulgated, through the Papal Bull “Inter Gravissimus”, his reformed calendar.  Rather than a four year leap year cycle, Gregory’s calendar counted 97 leap days in a 400 year cycle.  The three “ordinary” years in each cycle were years ending in “00” that were not evenly divisible by 400.  Under this scheme the years 1700, 1800, and 1900 were ordinary years, while the year 2000 was a leap year.  The length of a mean Gregorian year is 365.2425 days, so it’s still slightly longer (by an average of about 11 seconds) than the tropical year.  This will result in a one day error in a bit over 3000 years, but due to slow changes in the Earth’s orbit and the gradual slowing of the Earth’s rotation we probably won’t need to reform the current calendar for another 7000 years.  However, if we do reform the calendar, we will probably use the scheme promoted by Sir John Herschel in the 19th Century.  He proposed a 4000 year cycle that had 969 leap days, with millennial years evenly divisible by 4000 counted as ordinary years instead of leap years.  Fortunately we have plenty of time to contemplate this issue!

We are also entering the time of the year when the daily change in daylight reaches its peak rate.  As we enter March, the length of day increases by about three minutes per day.  Between now and the equinox on March 20th we’ll add a full hour of daylight to the 11 hours we’re experiencing now.

The winter stars still dominate the early evening hours, but by late evening the stars of Leo and Ursa Major herald the imminent arrival of spring.  If you follow the “arc” of the “handle” of the Big Dipper asterism, which is now prominently placed in the northeast, you’ll find a furiously flickering bright yellow-tinted star rising at around 9:00 pm.  This is the star Arcturus, the brightest star in the northern hemisphere of the sky, and the star that I have always associated with warmer evenings as winter loses its grip on the landscape.

Venus continues to climb higher in the west as she drives steadily northeastward along the ecliptic.  By the end of the week she is poised to move into the constellation of Aires, the Ram.  Over the course of the next month she will draw a bead on the Pleiades star cluster, passing through the cluster in early April.

Three bright planets may be found in morning twilight among the rising stars of Sagittarius.  Look low in the southeast for the ruddy glimmer of Mars, the bright cream-hued glow of Jupiter, and the yellow tint of Saturn.  Early in the week the distance between Mars and Saturn spans about 20 degrees, with Jupiter perched between them.  Mars will move eastward with each passing morning, and by next week the line of planets will span 16 degrees.  Over the course of the next few weeks Mars will catch up with Old Jove and pass the giant planet on March 20th.

The Sky This Week, February 18 – 25, 2020 !

The Sky This Week, 2020 February 18 – 25

Moonlight, mangers, and manes.
Orion and Sirius, 2014 March 27, imaged from Paris, Virginia.
Orion and Sirius, 2014 March 27, imaged from Paris, Virginia
with a Canon EOS Rebel T2i DSLR.

The Moon spends the week as a thin crescent in twilight, waning in the morning sky as the week opens and waxing in the evening sky as the week ends.  New Moon occurs on the 23rd at 10:32 am Eastern Standard Time.  Look for the very slender waxing crescent Moon low in the west half an hour after sunset on the 24th.  Luna cozies up to dazzling Venus on the evenings of the 26th and 27th.

The February Globe at Night observing campaign continues this week, with Orion as the featured constellation to view.  Orion crosses the meridian at 8:00 pm, which places him in the perfect position to count the number of stars you can see from your location.  It is also a constellation with stars that cover a wide range of brightness, so star counts can be easily made from the heart of the city or from the darkest sky you can find in rural areas.  All you need to do is find an unobstructed view of the sky away from the direct glare of street lights, let your eyes adapt to the darkness for about 15 minutes, then compare your view of Orion with star charts on the Globe at Night’s web app.  This “citizen science” program has now been going strong for over 10 years, and it has been instrumental in helping scientists measure the encroachment of artificial light on the night sky.  Data from the Globe at Night program has been a great help in establishing “dark sky parks” around the nation and the world, so please take a few minutes from your evening activities to help preserve our night sky.

As the evening grows later, the stars of winter are gradually being replaced by some of the rising stars of spring.  The easternmost stars of the Great Winter Circle cross the meridian by 10:00 pm in the form of the Twin Stars of Gemini, Castor and Pollux, and the star Procyon in Canis Minor.  To the east of Procyon is the star Regulus, the heart of Leo, the Lion, one of the more distinctive constellations of spring.  Between Leo, Gemini, and Canis Minor there is a scattering if faint 3rd- and 4th-magnitude stars that betray the constellation of Cancer, the Crab.  Normally this area of the sky wouldn’t attract much attention, but if you’re out at a good dark sky site you may notice a small, cloudy patch of light among the scattered stars.  Binoculars will show that this “cloud” is really a star cluster that was known to the ancient Greeks and Romans as the Praesepe, or “the manger”.  Today it is most popularly known as “the beehive”, and binoculars will reveal it to be a fairly rich swarm of faint stars.  Located at a distance of about 577 light-years, large telescopes have identified some 1000 stars as cluster members.  A modest amateur telescope will show several dozen of the cluster’s brighter members, and it is perennial favorite at springtime star parties.

Turning our attention to Leo, we find a large constellation that actually resembles its namesake.  North of Regulus, the Lion’s regal head and mane is formed by a semicircular arc of 2nd- and 3rd-magnitude stars, while his haunches are delineated by a right triangle asterism just over 15 degrees east of Regulus.  The constellation’s second-brightest star, Algeiba, lies just north of Regulus and shines with a delicate yellow hue.  Point a telescope at Algieba and you will see a fine double star consisting of a close pair of golden suns with about one magnitude difference in brightness.  This is a typical physical binary pair, with an orbital period that takes the stars around their center of mass in about 550 years.  It is one of my favorite objects to view in my 4-inch refractor.

Venus beams down brightly from the western sky during the evening hours.  She pops into view shortly after sunset, and once the sky gets fully dark it almost hurts your eyes to look at her.  From very dark locations she casts enough light to throw shadows, especially on a whiteboard on the ground.  She will welcome the waxing crescent Moon early next week.

Most of the planetary action takes place in the pre-dawn sky, where ruddy Mars and bright Jupiter cavort in the stars of summer constellations.  Mars drifts eastward over the top of the “teapot” asterism of Sagittarius.  By the week’s end Mars passes just under two degrees to the north of Kaus Borealis, the star that marks the teapot’s “top”.  Jupiter follows Mars and slowly drifts under the “teaspoon” asterism of faint stars in a view that should be a treat for binocular observers.

The Sky This Week, February 11 – 18, 2020 !

The Sky This Week, 2020 February 11 – 18

Mars hides behind the Moon, Betelgeuse bottoms out.
Messier 38, galactic star cluster in Auriga.  The smaller cluster is NGC 1907.
Messier 38, galactic star cluster in Auriga. The smaller cluster is NGC 1907.
Imaged with a Canon EOS Rebel T2i DSLR and Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor
on 2016 December 31 from Mollusk, Virginia.

The Moon wanes in the morning sky this week, diving through the springtime constellations to wind up the week among the early stars of summer.  Last Quarter occurs on the 15th at 5:17 pm Eastern Standard Time.  You’ll find the Moon several degrees north of the bright star Spica in the constellation of Virgo on the morning of the 13th.  If you’re up before sunrise on the 16th look for Luna just north of the stars that form the “head” of Scorpius.  On the morning of the 18th there’s a chance to see the Moon occult the planet Mars.  You will need to have a clear sky, a good unobstructed view of the southeast horizon, and a pair of binoculars or a small telescope to watch Mars slip behind the Moon’s bright limb at 7:28 am EST.  Sunrise will be at 6:51 am, so the sky will be fairly bright.  Mars emerges from behind Luna’s dark limb at 9:00 am, but this part of the event will be difficult to see.

In addition to being Valentine’s Day, the 14th marks the start of the February observing campaign for the Globe at Night citizen-science program.  The featured constellation for this month is Orion, which is perfectly placed in the early evening sky.  The folks at Globe at Night are particularly interested in your input for this campaign as they will be comparing current observations with those made back in 2012.  Since that year many outdoor lights have been converted to LED lamps, and astronomers are very interested in how this is changing the view of the night sky.  Making an observation is very easy.  Locate Orion in the sky, let your eyes adapt to the darkness, then compare your view with star charts located on the program’s website.  Your observations can make a difference in the long-term quality of the night sky.

Speaking of Orion, the star Betelgeuse, which usually rivals Rigel in brightness, continues to fade.  My most recent estimate places it on the same level as the brightest of the constellation’s “belt” stars.  The star has long been known to be variable in its brightness, but its current level is unprecedented in its observational history.  However, the big fade may be nearing an end.  Astronomers have determined that the star is experiencing a coincidence in two long-term variable cycles.  One of these is a well-known period of around 430 days, while the other seems to be about six years in duration.  Both cycles seem to be at their minima at this time, causing the star to appear dimmer than it has been for at least a century.  Both cycles should begin to “rebound” in the near future, and it is quite likely that Betelgeuse will begin to brighten by the beginning of March.  Watch this space!

If you are spending the holiday weekend away from city lights, take some time to locate the wintertime swath of the Milky Way.  While it is nowhere near as prominent as it is in the summer months, the faintly glowing band is still worth taking some time to explore.  Running from Cassiopeia in the northwest through Canis Major in the southeast, the soft glow of the Galaxy is peppered with star clusters that are some of the best celestial treats for binoculars and small telescopes.  One of my favorite regions to explore is contained within the pentagon shape of the constellation Auriga, which passes overhead at around 8:30 pm.  Three prominent clusters lie within the constellation’s outline, appearing as bright fuzzy knots in binoculars and resolving into swarms of dozens to hundreds of stars in small telescopes.  Sweep the Milky Way southeastward through the obscure constellation Monoceros, the Unicorn, which lies just east of Orion, and continue to the blazing star Sirius in Canis Major.  There are dozens of other star clusters and glowing clouds of gas to catch your careful eye.

Dazzling Venus appears high in the southeast as the Sun sets.  If you note where Venus appears in your local sky, try sighting her before sunset.  Once you know what to look for you may even be able to sight her when she crosses the meridian at around 3:00 pm.  If that’s too much of a challenge, go to a dark site and look for your shadow cast by Venus’ brilliant glow.

Look for ruddy Mars near the Moon before dawn on the 18th.  The red planet is drifting eastward above the stars that form the “teapot” asterism in Sagittarius, ending the week above the teapot’s “spout”.  As morning twilight begins to brighten the horizon, look for the bright glow of Jupiter between Mars and the southeast horizon.

The Sky This Week, February 4 – 11, 2020 !

The Sky This Week, 2020 February 4 – 11

Fading Betelgeuse, rising bear.
The Big Dipper rising over the Northern Neck, 2019 February 16, Mollusk, Virginia
The Big Dipper rising over the Northern Neck, 2019 February 16.
Imaged with a Canon EOS Rebel T2i DSLR and Omegon mechanical star tracker.
from Mollusk, Virginia.

The Moon waxes to her full phase this week, brightening the winter’s night sky as she passes high over the Great Winter Circle before moving into the rising stars of spring.  Full Moon occurs on the 9th at 2:33 am Eastern Standard Time.  February’s Full Moon is popularly called the Snow Moon, but she has a tough time living up to that name this year.  On the evening of the 5th and morning of the 6th, Luna occults two bright stars in the constellation of Gemini, the Twins.  Here in Washington the 3rd-magnitude star Propus disappears behind the Moon’s dark limb at 7:44 pm EST on the 5th, emerging from the bright limb at 8:35 pm.  At 12:45 am on the 6th the second-magnitude star Tejat goes behind the Moon near Luna’s southern cusp.  The star emerges at 1:09 am.  Observers along the southern limit of the occultation will have a chance to watch a spectacular “graze” as the star disappears and emerges from behind mountain peaks on the Moon’s rugged Limb.  In the Washington area the graze path runs from New Market, Virginia through Fredericksburg and across the Northern Neck.

The nearly-full Moon washes out most of the fainter stars in the sky, but the bright constellations of winter still give us familiar star patterns to enjoy.  By 8:30 pm the distinctive outline of Orion, the Hunter crosses the meridian.  Orion is one of the most recognized constellations in the heavens, visible from virtually all of the inhabited parts of our planet.  This year, however, the Hunter looks a little different than he has in decades.  Orion normally sports two bright first-magnitude stars, Betelgeuse and Rigel, which mark one of his shoulders and knees, respectively.  Normally the stars are fairly close in brightness, but for the past several months Betelgeuse has faded by over one full magnitude.  My most recent view of the star from the past weekend places it at a similar brightness to Alnitak and Alnilam, the left and center stars in Orion’s “belt”.  Betelgeuse has been known to vary in brightness for many years, but the current fading is unprecedented in the historical record.  How long it will continue to fade and how faint it will get is still anybody’s guess, as are the reasons behind the fading.  Betelgeuse is a very old, evolved star, so we may be witnessing the beginning of its ultimate demise into an intense supernova explosion, which could happen this week or 100,000 years from now.  For those of us who think of the stars as “immutable”, this is a fine example that demonstrates the idea of the Universe as a vast evolving system.

The stars of the Great Winter Circle cross the meridian as the evening hours pass.  Looking to the east and northeast we find some of the signature constellations of spring making their way into the sky.  Leo, the Lion, led by its first-magnitude star Regulus, is fully above the eastern horizon by 9:30 pm.  The Full Moon will pass close to Regulus on the evening of the 9th, then move below the Lion’s tail on the following night.  Leo consists of two prominent asterisms.  The first is known as the “Sickle” and consist of Regulus and a semi-circle of second- and third-magnitude stars that sit north of the bright star.  The second asterism lies east of the Sickle and consists of a compact right triangle.

Looking to the northeast you should be able to easily spot the seven stars that make up the “Big Dipper” asterism, part of the larger constellation of Ursa Major, the Great Bear.  The Dipper is a striking sight at this time of the year, seemingly standing on its “handle”, balancing the “bowl” above.

Venus remains as the most prominent planet in the evening sky.  She continues to gradually climb northward along the ecliptic in the western sky, while at the same time moving farther from the Sun.  You have plenty of time to enjoy her brilliant glow since she doesn’t set until around 9:00 pm local time.

Early risers should be on the lookout for more planets in the pre-dawn sky.  Mars shines from the area of the sky between the constellations of Scorpius and Sagittarius, now cresting the southeast horizon as morning twilight gathers.  If you have a clear view of the southeast horizon, scan about 10 degrees above the skyline for the bright glow of Jupiter.  The giant planet should be easy to spot by 6:00 am, and you should see a nice contrast between the whitish hue of Old Jove and the pink hue of Mars about 20 degrees to the west.

The Sky This Week, January 28 – February 4, 2020 !

The Sky This Week, 2020 January 28 – February 4

Lunacy and groundhogs
The Moon near First Quarter phase, 2016 November 7, imaged at the U.S. Naval Observatory
The Moon near First Quarter phase, 2016 November 7, 22:42 UT
Imaged with a Canon EOS Rebel T2i DSLR and USNO’s 30.5-cm (12-inch)
Clark/Saegmüller refractor.

The Moon waxes from crescent to First Quarter this week, ending the week among the bright stars of the Great Winter Circle.  First Quarter occurs on February 1st at 8:42 pm Eastern Standard Time.  Look for Luna passing between the bright star Aldebaran and the Pleiades star cluster on the evening of the 3rd.

This is a good week to dust off the telescope and explore our only natural satellite.  As the Moon waxes she climbs higher along the ecliptic, placing her well above the horizon for telescopic perusal. A higher altitude means that there’s less of our own atmosphere to look through, and typically this allows for sharper rendering of the Moon’s finer details.  Don’t expect to see any of the flags left at any of the Apollo landing sites, though.  Under the best of circumstances a telescope such as our venerable 12-inch refractor won’t show features much smaller than a kilometer or so.  That said, there’s a great deal of detail that can be glimpsed on Luna’s desolate surface from your own yard with a modest instrument.  I particularly like the days just before and after First Quarter when the slow-moving terminator line reveals the battered southern “highland” area.  Here you will find mute testimony to the violent formation of the solar system, with impact craters packed one on top of another.  This surface is one of the oldest we see in the solar system and reflects an age of intense bombardment of newly-formed planets by thousands of “planetessimals”.  This period has largely been wiped from Earth’s geological record by plate tectonics and erosion, but on the airless Moon it is a “window” on conditions that existed over four billion years ago.

February 2 is a date that is widely celebrated as “Groundhog Day” where, according to legend, we rely on the prognostication power of a large indigenous rodent of the marmot family to tell us how long winter will last.  According to the legend, if the groundhog sees its shadow on that fateful morning, winter will last for another six weeks.  Statistically the groundhog has nearly a 100 percent chance of being correct, since the vernal equinox falls just over six weeks from the 2nd.  However, astronomical seasons don’t necessarily correspond to meteorological ones, so the groundhog’s “guess” is about as good as anybody else’s this far in advance.  While we may think of Groundhog Day as a quaint observance by a group of dapper-dressed folks in a small Pennsylvania town, it is actually a traditional astronomical observance whose roots date back over a thousand years.  As it turns out, Groundhog Day is what was once known as a “cross-quarter” day, one of eight traditional days when feudal serfs paid their land-owner masters.  These dates were determined by the astronomical seasons as defined by the equinoxes and solstices.  These annual markers divided the year into quarters, and the dates halfway between these markers further divided the year into eighths.  We are all familiar with the quarter days, but the cross-quarter days have become a bit more obscure.  However, three of these dates still linger in our modern culture, so if you observe Groundhog Day, May Day, and Halloween, you are unwittingly carrying on the tradition.  The fourth cross-quarter day, Lammas, which falls on August 1st, seems to have vanished in American folklore, but in many European cultures it is the day to start one’s traditional summer vacation.

Venus keeps an eye on the waxing Moon as the latter pulls away in the evening sky.  The pair are close together at dusk on the 28th, then Venus resumes her lonely vigil in the sparse constellations of the autumn sky.  The dazzling planet continues to pull away from the approaching Sun as she moves northeastward along the ecliptic, and she now sets almost two hours after the end of evening astronomical twilight.  You should have no trouble spotting her in the west shortly after sunset.

Mars now rises at around 4:00 am local time, and as morning twilight gathers you can find the red planet low in the southeastern sky.  Mars is steadily moving eastward along the southernmost part of the ecliptic, passing through constellations that we associate with the summer sky.  If you’re up at around 6:30 am and have a clear view to the southeast start looking for the bright glimmer of Jupiter.  Mars is drawing a bead on the giant planet, and early risers in mid-March will get a great view as Mars passes his distant companion.

The Sky This Week, January 21 – 28, 2020 !

The Sky This Week, 2020 January 21 – 28

What’s dimming Betelgeuse?
Orion as seen from the suburbs of Washington, DC, 2020 January 22
Orion as seen from the suburbs of Washington, DC, 2020 January 22
Imaged with a Canon EOS Rebel SL2 DSLR and an Omegon mechanical star tracker
from Alexandria, Virginia.
The right image is de-focused to show the colors and relative brightnesses of the
constellation’s brighter stars. Note the brightness of Betelgeuse compared to that
of Bellatrix and Rigel.

The Moon starts the week off as a waning crescent in the pre-dawn sky and ends as a waxing crescent by the week’s end.  New Moon occurs on the 24th at 442 pm Eastern Standard Time.  Look for the Moon in the company of bright Venus after sunset on the evenings of the 27th and 28th.

As mentioned last week, the long winter nights are dominated by the stars of the Great Winter Circle and centered on the prominent outline of Orion.  This was the first constellation that I learned when I was very young, dominating the winter sky over my home in rural New England.  His three “Belt Stars” immediately caught my eyes as did the exceptional ruddy tint of Betelgeuse, whose contrast with the blue tints of Orion’s other bright stars fascinated me.  Since that time I have always regarded Betelgeuse as similar in brightness to Rigel, Orion’s other prominent star, and often I thought that Betelgeuse was slightly brighter.  Until recently.  When I viewed the Hunter in September from West Virginia all seemed normal, with Rigel and Betelgeuse sharing the mantle of the constellation’s brightest stars, but since then Betelgeuse has pulled something of a vanishing act.  When I looked at Orion from my suburban yard last night, I found Betelgeuse to be slightly dimmer than the star Bellatrix, which marks Orion’s opposite “shoulder”.  This means that Betelgeuse has faded by a full magnitude since September, and the star is now dimmer than at any time since reliable brightness measurements have been kept.

Many stars in the sky are known to vary in brightness.  Some, like Algol, the “Demon Star” in Perseus, were duly observed to vary by ancient Arabic astronomers.  Algol’s brightness changes are very predictable, and the star fades from second- to third-magnitude every 2.9 days.  The reason is simple: Algol is a binary star, and once every cycle the brighter star is partially obscured by the fainter companion.  Hundreds of these “eclipsing binaries” are scattered across the sky.

Another type of variable star also fades and brightens to a set rhythm.  These stars, known as Cepheid variables after their prototype Delta Cephei, go through regular pulsations with periods lasting from days to months.  Their brightness changes are caused by fluctuations of ionized helium in their outer layers.  They are intrinsically bright, and there is a well-documented correlation between their periods of oscillation and their luminosity.  Because they are so bright, we can use them to measure distances to many nearby galaxies.

Betelgeuse, on the other hand, is a “semiregular” variable star that displays two or possibly more variation modes, one of which is around 425 days and the other around 6 years.  It typically oscillates between magnitudes 0.5 to 1.3.  I have often seen it comparable to Rigel in brightness, but I have never seen it as dim as it is right now.  Explaining the mechanism of this variability is a challenge for astronomers because Betelgeuse is a very unusual star.  It is very old for a star, closing in rapidly (in cosmic terms) to its demise.  Internally it has depleted the hydrogen and other light elements in its core and now fuses these elements into heavier ones in onion-like layers surrounding a core of iron.  From a nuclear point of view, iron is inert, since it takes more energy to fuse it onto something heavier or split it into smaller nuclei than such a reaction would release.  This “shell-burning” phase has caused its outer layers to swell to a size comparable to the orbit of Mars around the Sun.  These layers are subject to upwelling of large convective cells of hot gas, and these cells may be the mechanism that cause the changes in the star’s brightness.  Most astronomers think that Betelgeuse will rebound from this deep minimum, but when that minimum actually will occur is anybody’s guess.  Watch the star, and watch this space.

Dazzling Venus continues to climb higher in the evening twilight sky.  She should be easy to pick out in the southwest shortly after sunset, and if you know where to look you should be able to spot her before the Sun goes down.  She is steadily climbing northward along the ecliptic, and by early February she will enter the northern hemisphere of the sky.

Ruddy mars may be seen in the gathering morning twilight, low in the southeastern sky.  He is pulling away from the bright star Antares in Scorpius as he skims along the southernmost reaches of the ecliptic.  This will be a good year for the red planet.  When he comes to opposition in the fall he will be well placed for northern observers and rival Jupiter in brightness.

The Sky This Week , January 7 – 14, 2020 !

The Sky This Week, 2020 January 7-14

A fading giant in Orion
Orion over the Northern Neck, New Year's Eve 2019-2020
Orion over the Northern Neck, New Year’s Eve 2019-2020
Imaged with a Canon EOS Rebel SL2 DSLR from Mollusk, Virginia, 2020 January 1.
Note the brightness of Betelgeuse compared to that of Rigel. Normally the
two stars are comparable in brightness.

The new year finds a waxing Moon high in the sky, brightly lighting the stark winter terrain below.  Full Moon occurs on the 10th at 2:21 pm Eastern Standard Time.  January’s Full Moon is popularly known as the Wolf Moon or Moon After Yule.  Skywatchers in Europe and Asia have the opportunity to see a penumbral lunar eclipse at this time.  That said, the effect on Luna’s disc is very subtle.  Observers may notice a slight dimming of the Moon’s southern limb, but none of her disc will touch the darker umbral shadow of the earth.  Don’t plan to take a last-minute trip abroad to catch it!  Look for the Wolf Moon between the stars Castor and Pollux, the Gemini Twins, and Procyon, brightest star in the constellation Canis Minor, the Little Dog.  By the end of the week she treks across the stars of Leo, the Lion.

With the winter solstice a couple of weeks behind us, the days are now gradually getting longer.  This is especially noticeable at the time of sunset.  On the evening of the 11th Old Sol dips below the horizon at 5:06 pm EST, some 20 minutes later than his earliest setting time back on December 7th.  By the end of the month sunset will approach 5:30 pm.  Spring is on the way!

The bright Moon once again washes out much of the night sky, but winter’s bright constellations are pretty hard to hide in her glow.  The most prominent of the season’s constellations is Orion, the Hunter, whose distinctive outline is familiar to almost all of the Earth’s inhabitants.  This year is shaping up to be an especially good year to check out Orion on a nightly basis, since one of his most prominent stars has been behaving strangely.  Normally the star that marks the Hunter’s left “shoulder”, the red-tinted Betelgeuse, shines with about the same apparent brightness as the blue star Rigel, one of Orion’s “knees”.  However, a casual glance at the constellation will show that Betelgeuse is barely brighter than any of the Hunter’s three “Belt” stars.  Betelgeuse has long been known to vary in brightness, but it hasn’t been this faint for well over a century.  Betelgeuse is, however, a very unusual star.  It is classified as a “red super-giant”, a massive star that has evolved through most of its life cycle.  Its interior is layered like an onion, with different layers fusing heavier elements in shells surrounding an iron-rich core.  As these shells expand they cause the outer convective layers of the star to swell to immense proportions; some estimates indicate that its diameter is larger than the orbit of Mars around our Sun.  These tenuous outer layers are relatively cool, giving Betelgeuse its characteristic red color and allowing the formation of molecular and “dust” layers.  Changes in the opacity of these outer layers may be the mechanism that causes in the star’s apparent brightness to fluctuate.  On the cosmic scale Betelgeuse is in a very unstable state, and it is one of the leading candidates for becoming a supernova in the near (i.e. sometime in the next 100,000 years!) future.  We don’t know when the current dimming trend will stop, but in all likelihood Betelgeuse will probably brighten back to its “normal” luster over the next few years.  For now, you can do a little “citizen science” by comparing the brightness of Betelgeuse to the other prominent stars in Orion and charting your results.

Venus is now the only bright planet gracing the evening sky.  You will find her high in the southwest shortly after sunset, blazing away among the dim stars of the autumnal constellations.  She sets at around 8:00 pm EST.

You’ll have to wait until the pre-dawn hours to spot the next visible planet.  Mars rises shortly after 4:00 am along with the first of summer’s stars.  This week the red planet is moving through the stars of Scorpius, and by the week’s end he will be within a few degrees of the star Antares, another red supergiant star whose name means “rival of Mars”.  Antares and Betelgeuse are similar in their sizes, distances, and evolution, but you will never see them in the sky together.  In one of the myths surrounding Orion, he was killed by the sting of a scorpion, so the two are placed on opposite sides of the sky, never to encounter one another again.

The Sy This Week, December 20, 2019 – January 7, 2020 !

The Sky This Week, 2019 December 20 – 2020 January 7

Time for a long winter’s nap, with apologies to Clement Clark Moore!
Happy Holidays from the U.S. Naval Observatory!
Happy Holidays from the U.S. Naval Observatory!

‘Tis the Night Before Christmas and up in the dome

We eagerly wait for the nightfall to come.

The slit has been opened, the lens cap’s been stowed

The night sky awaits like a wide-open road.


The solstice will fall on the 21st day,

The Sun’s southernmost point on his orbital way.

The year’s longest nights are upon us right now

But they start to get longer when the Yule log’s aglow.


The Moon is now waning through spring’s rising stars

Her slimming crescent will soon pass by Mars.

New Moon occurs as Boxing Day falls

Then fills out to her crescent as New Year’s Night calls.


Bright Venus shines down in the twilight’s last glow,

Her dazzling brilliance a shadow can throw.

As she climbs through the stars of the Sea-Goat’s sparse sky

She’ll stay bright in the evenings ‘til summer draws nigh.


The Great Winter Circle shines bright in the night

With bright stars a-twinkling with all of their might.

Their colors add contrast to enhance the dark sky

While far down below they’re a treat for the eye.


Orion is rising high in the southeast,

Shield raised in defiance of Taurus the beast.

The Great Winter Circle surrounds his bold shape,

While faithful dog Canis leaps up in his wake.


Late night brings Sirius, the Dog Star on high,

By New Year’s he transits as midnight draws nigh.

The brightest of stars warm the long winter’s night,

His cohorts all add to the breathtaking sight.


Nine of the brightest of stars in the sky,

Light these long nights of winter as Old Sol plays shy.

With the solstice upon us we’ll all soon be glad,

For the days getting longer than the ones we’ve just had.


The stars of the springtime rise late in the night

With a singular planet to cause you delight.

Red Mars moves southeastward toward Antares’ glow

He’ll pass north of his rival next month as he goes.


The first stars of summer rise just before dawn

With the ringed Old Jove and Saturn soon tagging along.

These bright worlds will be with us all summer next year

A treat to look out for when warm nights are here.


So Peace to your families, neighbors, and friends

We wish you the best that the holiday sends.

The stars mark the comings and goings of time,

So stop to enjoy them, and so ends my rhyme.

The Sky This Week, December 10 – 17, 2019 !

The Sky This Week, 2019 December 10 – 17

Brightening the long winter night.
Orion rising over the Catalina Mountains, imaged from Catalina, Arizona, 2019 January 14.
Orion rising over the Catalina Mountains
imaged from Catalina, Arizona, 2019 January 14,
made with a Canon EOS Rebel SL2 DSLR.

The evening hours are dominated by the bright light of the Moon for most of the week as she rides high along the ecliptic through the heart of the Great Winter Circle.  The Full Moon Before Yule occurs on the 12th at 12:12 am Eastern Standard Time, then wanes to last Quarter on the 18th.  On the evening of the 10th she glides just north of the Hyades star cluster near the bright star Aldebaran, the “eye” of Taurus, the Bull.  By the end of the week Luna turns southward in her course, passing just over three degrees north of the star Regulus in Leo, the Lion.

You probably won’t notice it, but starting on the 13th the time of sunset will gradually start to shift a bit later.  Our earliest sunsets now occur at 4:46 pm here in Washington, and by the time that Christmas rolls around Old Sol will set six minutes later than he does now.  That said, we are still losing daylight in the mornings, as our latest sunrise won’t occur until early next year.  However you look at it, though, we are now experiencing the longest nights of the year.

Fortunately, when the Sun is shy and scudding over the southern horizon, the Moon reaches her highest declinations for her full phase for the year, nestled among a bevy of the sky’s brightest stars.  Once the Moon leaves the scene, those bright winter stars linger on, adds at least a little extra light to the longest nights.  Dominating this group of stars is the distinctive figure of Orion, the Hunter.  Of all the constellations of the season, Orion stands out despite the best efforts of the Moon to wash out his distinctive features.  Although they are not the constellation’s brightest stars, the three stars that mark Orion’s “Belt” are almost universally synonymous with the boreal winter sky.  They were one of the first star patterns that I noticed at a very early age, and Orion was the first constellation I explored with my new Christmas-present telescope.

A casual look at Orion with the naked eye shows that most of the Hunter’s stars are tinted blue, and this betrays their common origin in a vast star-forming region that’s centered on the Great Orion Nebula in the asterism known as “The Sword”.  This small clump of stars hangs below the left side of the bright Belt stars, and if you look at the group with binoculars you will immediately see that the middle star in the group has a fuzzy appearance.  A small telescope will show a tight knot of four stars surrounded by wisps of glowing gas even under urban skies and bright moonlight.  Move to a dark sky site on a moonless night and the extent of the nebulosity grows, filling the eyepiece with tendrils of soft diffuse light.  The stars that have formed in this cloud now make up many of the brighter stars in Orion.  They are very far away and thus very luminous.  The bright blue stars that dominate the constellation are well over 1000 light-years away, and to appear that bright over such a vast distance they shine with the luminosity of tens of thousands of Suns!

The one exception is perhaps the most distinctive star in the constellation, Betelgeuse, which marks Orion’s right shoulder.  You will immediately notice its reddish tint, far different from its bright companions.  Betelgeuse is located at about half the distance to the rest of the blue stars in Orion, and while the blue stars are comparatively young in their evolutionary age, Betelgeuse is nearing the end of its stellar life.  It is a “red supergiant” star that has exhausted the hydrogen in its core, and it now fuses hydrogen into helium in an ever-expanding shell around that core.  This causes the star’s girth to expand dramatically, and if there were a planetary system analogous to our solar system around the star all of the planets out to the orbit of Mars would be swallowed by its outer layers!   Betelgeuse is also an irregular variable star whose light changes by over a full magnitude over the course of several years.  At times it is brighter that its rival Rigel, the star that marks the Hunter’s left knee.  This year, however, it is decidedly fainter than Rigel and may be on the way to its faintest minimum since 1941.

Venus may now be easily seen in the evening twilight sky.  As the sky darkens on the evenings of the 10th and 11th you will see the planet Saturn just to the north of the dazzling planet.  Venus will quickly abandon her more distant companion leaving Saturn to set at the end of evening twilight.  Venus will continue to climb in the evening sky and will be our evening companion until late in the coming summer.

Ruddy Mars can be spotted before sunrise low in the southeastern sky.  On the morning of the 12th he will pass by the wide double star Zubenelgenubi in the constellation of Libra, the Scales.  In binoculars Mars and the star will almost appear to touch, and they will be close enough to each other to appear in the same field of view of a telescope.  This will be the closest appulse of a planet and star for the year, so try to catch the sight if you can.