The Night Sky This Week : July 20 – 27, 2010 !

The Sky This Week, 2010 July 20 – 27

Planets converge in the evening, Jupiter moves into the evening sky

Sunset, Morattico, Virginia, 2010 July 17

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The Moon hugs the southern horizon this week, drifting through the stars of Scorpius and Sagittarius before venturing into the relatively empty regions of the autumnal constellations.  Full Moon occurs on the 25th at 9:37 pm Eastern Daylight Time.  July’s Full Moon is popularly known as the Hay Moon or Thunder Moon, and the latter name seems particularly appropriate this year.  Luna starts the week off hovering among the stars at the head of Scorpius, the Scorpion.  On the 21st she shines just east of the ruddy star Antares.  On the 23rd she sits just above the teapot-shaped asterism that forms the brightest part of Sagittarius, and by the week’s end she sets her sights on bright Jupiter, low in the east at the midnight hour.

The Moon’s southerly declination, coupled with high humidity and atmospheric haze, will render most of the starry sky very difficult to discern.  This is further compounded by the reflected glare of urban streetlights, which cast their green and orange-tinted palls over the entire sky.  There have been a number of evenings of late when I have found it almost impossible to sight the planets Mars and Saturn, even though they are both first magnitude objects, from my suburban home.  However, there are a few bright objects that can cut through the haze and glare other than the bright planets.  Most notable of these is the star Arcturus, which now hangs high in the western sky as twilight ends.  Arcturus is the brightest star in the northern hemisphere of the sky, and the third-brightest star in the heavens.  Its rosy glow is a fixture in the spring sky, but as summer progresses the star becomes overshadowed by other bright luminaries in the night.  At about the same altitude in the east is the fifth-brightest star in the sky, the blue-tinted luminary called Vega.  Both of these stars are bright because they are relatively close to us in space.  Arcturus lies at a distance of about 37 light-years, while Vega is practically around the corner at 25 light years.  Arcturus holds a singular place in the sky, but Vega has two bright companions that form a large distinct asterism known as the Summer Triangle.  Together with the stars Deneb and Altair, Vega and its companions point in a southerly direction where you might find the ruddy star Antares if it’s clear enough and the second magnitude stars that comprise the “Teapot” of Sagittarius if it’s clear enough.  It’s always good practice to identify these basic stars in a bright sky.  Once the Moon goes away and you head to that dark vacation spot you’ll now have some convenient guideposts to steer you toward more interesting sights!

The late twilight of the evening sky finds the beginnings of a planetary “traffic jam” in the southwest.  Venus will be the most obvious planet as the light of early evening fades, and she is steadily closing in on ruddy Mars and gold-hued Saturn.  I happened to view Venus shortly before she set last weekend from down on the Northern Neck of Virginia.  Thanks to a flat horizon, thin clouds, and haze, the normally dazzling white planet was glowing like a single orange-red coal in a dying campfire.  She spends the week in restless pursuit of Mars and Saturn, which are gearing up for their own conjunction next week.  

The late night sky now welcomes the bright glow of Jupiter, who is doggedly rising about four minutes earlier each night.  Old Jove reaches the first stationary point in the current apparition on the night of the 23rd, pausing for a few days in his eastward motion before seeming to back up toward the west over the course of the next four months.  Late night skywatchers are now enjoying the view of the giant planet in their telescopes.  Soon he’ll be delighting even those of us with early bedtimes.

Visit the USNO : U.S. Navy Link for More Facts.

Phases of the Moon and Percent of the Moon Illuminated !

 

Phases of the Moon. Copyright Antonio Cidadao . Used by permission.

Note: For Moon phase information specific to a particular date, see

• Dates of Primary Phases of the Moon
• Complete Sun and Moon Data for One Day
• Fraction of the Moon Illuminated

What the Moon Looks Like Today

in Data Services.

From any location on the Earth, the Moon appears to be a circular disk which, at any specific time, is illuminated to some degree by direct sunlight. Like the Earth, the Moon is a sphere which is always half illuminated by the Sun, but as the Moon orbits the Earth we get to see more or less of the illuminated half. During each lunar orbit (a lunar month), we see the Moon’s appearance change from not visibly illuminated through partially illuminated to fully illuminated, then back through partially illuminated to not illuminated again. Although this cycle is a continuous process, there are eight distinct, traditionally recognized stages, called phases. The phases designate both the degree to which the Moon is illuminated and the geometric appearance of the illuminated part. These phases of the Moon, in the sequence of their occurrence (starting from New Moon), are listed below.

 

Moon Phases

 

New Moon – The Moon’s unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).

Waxing Crescent – The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is increasing.

First Quarter – One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is increasing.

Waxing Gibbous – The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is increasing.

Full Moon – The Moon’s illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.

Waning Gibbous – The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is decreasing.

Last Quarter – One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is decreasing.

Waning Crescent – The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is decreasing.

Following waning crescent is New Moon, beginning a repetition of the complete phase cycle of 29.5 days average duration. The time in days counted from the time of New Moon is called the Moon’s “age”. Each complete cycle of phases is called a “lunation”.

Because the cycle of the phases is shorter than most calendar months, the phase of the Moon at the very beginning of the month usually repeats at the very end of the month. When there are two Full Moons in a month (which occurs, on average, every 2.7 years), the second one is called a “Blue Moon”. See the article “Once in a Blue Moon” for the story of how the usage of this term has evolved (Ref: Philip Hiscock, Sky & Telescope, March 1999, pp. 52-55.). 

The first time that the thin waxing crescent Moon is visible after New Moon (low in the evening sky just after sunset) marks the beginning of a month in the Islamic Calendar – see the FAQ Crescent Moon Visibility and the Islamic Calendar.

Although Full Moon occurs each month at a specific date and time, the Moon’s disk may appear to be full for several nights in a row if it is clear. This is because the percentage of the Moon’s disk that appears illuminated changes very slowly around the time of Full Moon (also around New Moon, but the Moon is not visible at all then). The Moon may appear 100% illuminated only on the night closest to the time of exact Full Moon, but on the night before and night after will appear 97-99% illuminated; most people would not notice the difference. Even two days from Full Moon the Moon’s disk is 93-97% illuminated.

New Moon, First Quarter, Full Moon, and Last Quarter phases are considered to be primary phases and their dates and times are published in almanacs and on calendars. (Click here for a list.) The two crescent and two gibbous phases are intermediate phases, each of which lasts for about a week between the primary phases, during which time the exact fraction of the Moon’s disk that is illuminated gradually changes.

The phases of the Moon are related to (actually, caused by) the relative positions of the Moon and Sun in the sky. For example, New Moon occurs when the Sun and Moon are quite close together in the sky. Full Moon occurs when the Sun and Moon are at nearly opposite positions in the sky – which is why a Full Moon rises about the time of sunset, and sets about the time of sunrise, for most places on Earth. First and Last Quarters occur when the Sun and Moon are about 90 degrees apart in the sky. In fact, the two “half Moon” phases are called First Quarter and Last Quarter because they occur when the Moon is, respectively, one- and three-quarters of the way around the sky (i.e., along its orbit) from New Moon.

The relationship of the Moon’s phase to its angular distance in the sky from the Sun allows us to establish very exact definitions of when the primary phases occur, independent of how they appear. Technically, the phases New Moon, First Quarter, Full Moon, and Last Quarter are defined to occur when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0, 90, 180, and 270 degrees, respectively. These definitions are used when the dates and times of the phases are computed for almanacs, calendars, etc. Because the difference between the ecliptic longitudes of the Moon and Sun is a monotonically and rapidly increasing quantity, the dates and times of the phases of the Moon computed this way are instantaneous and well defined.

The percent of the Moon’s surface illuminated is a more refined, quantitative description of the Moon’s appearance than is the phase. Considering the Moon as a circular disk, the ratio of the area illuminated by direct sunlight to its total area is the fraction of the Moon’s surface illuminated; multiplied by 100, it is the percent illuminated. At New Moon the percent illuminated is 0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%.

For practical purposes, phases of the Moon and the percent of the Moon illuminated are independent of the location on the Earth from where the Moon is observed. That is, all the phases occur at the same time regardless of the observer’s position.

Lunation Movie

Small movie (134k)

If you click on either of the two images on the right, you will see a time-lapse movie of the appearance of the Moon over one lunation. A lunation is a lunar month, during which time the Moon completely circles the Earth in its orbit. The complete cycle of phases is obvious. Two other effects can be easily seen. First, due to the elliptical shape of the Moon’s orbit, the apparent size of the Moon’s disk changes as its distance from Earth varies (the closest and farthest points do not always occur at the same phases, however). Second, although the Moon’s near side directly faces the Earth on average, we get to view the Moon from slightly different angles as it orbits us. This effect, called libration, is caused partly by the tilt of the Moon’s rotation axis with respect to its orbital plane and partly by the fact that the Moon’s speed in its orbit varies but its rotation rate does not.

Large movie (493k)

The movie is an animated GIF created by Antonio Cidadao from a sequence of still images taken during March and April 1998.

Copyright Antonio Cidadao. Used by permission. 

The Night Sky This Week : July 13 – 20, 2010 !

The Sky This Week, 2010 July 13 – 20

The Moon passes a passel of planets…

Saturn, 2010 July 3, 01:58 UT Imaged at the U.S. Naval Observatory with the 1895-vintage 12-inch f/15 Clark/Saegmüller refractor

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The Moon, fresh from her rendezvous with the Sun over the South Pacific last weekend, returns to the evening sky this week.  First Quarter occurs on the 18th at 6:11 am Eastern Daylight Time.  During the week Luna’s waxing crescent pays nightly visits to a string of bright stars and planets, making the early evening sky the place to watch most of the week’s action.  On the evening of the 14th look for the Moon to the southeast of the brilliant planet Venus as twilight gathers.  On the 15th she has shifted to a position some six degrees south of ruddy Mars.  The 16th finds Luna about ten degrees below golden Saturn, while the 17th and 18th find her passing the vicinity of the bright star Spica.  She ends the week drifting through the stars in the “head” of Scorpius, drawing a bead on the red-tinted star Antares.  These close approaches should offer some fine opportunities for budding astrophotographers to hone their sky imaging skills.

The first few evenings of the week offer a chance to enjoy some of the splendors of the summer Milky Way once the Moon sets.  Clear skies and distance from city lights are necessary to see the Galaxy in all its glory, but a summer trip to the mountains or the shore can often result in both conditions being met.  The basic tools for a successful evening’s observing session are binoculars and bug repellent.  Binoculars are a great way to explore the vast star clouds of the Milky Way as well as its embedded star clusters and nebulae.  As you sweep over the denser portions of the galaxy above the “teapot” asterism of the constellation Sagittarius, you may notice sudden voids in the countless tracts of stars.  These seeming “holes” in the sky are actually clouds of dark material that obscure the light of more distant stars.  Well over 300 of these dark regions were catalogued in the early 20th Century by the American astronomer Edward Emerson Barnard, a pioneer in astronomical imaging of the Milky Way.

In the early evening sky no fewer than five bright objects (not counting the Moon) vie for your attention this week.  Venus will be the first to catch your eye, appearing in the bright glow of twilight almost immediately after the Sun has set.  Early in the week she’s located a few degrees east of the bright star Regulus, lead star in the constellation of Leo, the Lion.  You’ll probably need to wait until an hour after sunset to see the star, but once you find it you can watch Venus add another degree of separation each succeeding night.  To the east of Venus is the orange-tinted glimmer of Mars, which has the same apparent brightness as Regulus but sports a very different color.  By the end of the week Venus will be about halfway between Mars and Regulus in the fading twilight.

Ruddy Mars is also moving briskly to the east, closing in on yellow-hued Saturn.  During the course of the week Mars halves the distance between himself and the ringed planet, narrowing the gap from ten to five degrees.

Saturn is very gradually creeping eastward as well, but his nightly progress is almost impossible to detect.  By this time next year he will have only ventured as far as the star Spica, the last of our bright evening line-up.

Giant Jupiter now rises before midnight, and late-night skywatchers can begin to train telescopes on him by around 1:30 am once he’s climbed above the haze that typically shrouds the horizons on most warm summer nights.  Old Jove will offer a refreshing sight to those of us who have watched the smaller discs of the evening planets over the past several months.  The small telescope owner will be treated to a constantly changing surface of turbulent cloud tops, along with the four bright moons discovered by Galileo 400 years ago.

Visit My USNO : U.S. Navy Link for For Facts !