Astronomy: The Week Ahead – Sun 15 Aug to Sat 22 Aug 2015

aug sky photo


Sunday August 16

This evening at sunset the planet Mercury is directly west, and very close to the horizon as the sky becomes dark enough to try finding it. A two day old moon will accompany Mercury six degrees to its east, showing only a very thin 6% illuminated waxing crescent. You will be lucky to pick out either, as an absolutely flat horizon and good timing will be necessary. Mercury’s due west position will be helpful in spotting its magnitude 0.17 speck, while only the thin crescent of the moon will be possible. Above Mercury, Leo’s brightest tail star, Denobola, might be visible as the sky continues to darken.


Monday August 17

The dim and long constellation Eridanus, the River, is rising west of Orion in the predawn sky. You can easily locate its beginning, next to the bright blue-white giant star Rigel, at the foot of Orion. Like an old river, the path of Eridanus meanders through mostly empty skies, terminating due south below the horizon with the 0.5 magnitude star Achernar. Eridanus is the Latin name for the Po River, in Italy. It is an ancient constellation, among the 48 original ones designated by Ptolemy in the 2nd century A.D. Only four of its main stars shines brighter than magnitude 3.0.


Tuesday August 18

Today the moon enters ascending node, crossing north of the celestial equator, called the Ecliptic (green line). It is also now at apogee, its farthest point from earth in its orbit. Watch as it passes Spica tomorrow night, while its waxing crescent phase increases from 17% illumination, and passes Saturn to reaches first quarter phase with 53% illumination in four nights.


Wednesday August 19

Grab your binoculars and look for Mars low over the eastern horizon below Gemini. If you are able to see it, your binoculars should reveal the red planet shining brightly among the stars of the great open cluster called The Beehive, or Messier 44 (M44), in the constellation Cancer. Mars will be at magnitude 1.75, and easy to identify. The Beehive Cluster is magnitude 3.7, large and coarse, with many bright stars overflowing most binocular fields of view. Mars will be 48 arc-minutes from the center of the cluster this morning. The Beehive will overflow most binocular fields of view, at 95 arc-minutes in size.


Thursday August 20

Use your binoculars this morning to find the Great Andromeda Galaxy, M31, high in the western sky. The Square of Pegasus will be your starting point. Find it then hop up the gentle arcs of stars to Beta Andromedae, and scan across past the dimmer star (Mu Andromedae) just to its north, until you see a large glowing oval. You’ll have found the galaxy, or at least part of it. M31 is huge in our skies. At 178×63 arc-minutes, it is wide a five full moons and twice as thick! You won’t be able to view the entire galaxy in one binocular field of view! This galaxy at magnitude 3.4, is the farthest object that can be seen without optical aid, at a distance of 2.9 million light years.

Enjoy Saturn season while you can, it always seems to end too soon!


Friday August 21

Saturn reaches eastern quadrature today. The earth and Saturn form a right angle with the sun. We are now halfway through “Saturn season”, with the earth speeding away from Saturn, to leave it in the glare of the sun in a few more months. Since we are at right angles to the sun, you’d expect the distances to reflect that. We are currently 9.9 Astronomical Units (AU) from Saturn, and Saturn is 9.9 AU from the sun!


Saturday August 22

The first quarter moon and Saturn sits just under 5-1/2 degrees apart tonight, straddling the constellations Scorpius (moon) and Libra (Saturn). Use your extended three middle fingers, held at arms length, and they should just fit between these two celestial objects. Your fist equals about five degrees, so now you have a measuring device at hand! A fist is ten degrees. The moon tonight is so bright it will drown out our view of the Milky Way rising from the Teapot is Sagittarius. Surprisingly, it is only 1/11th as bright at a full moon!

Happy viewing!



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