The Sky This Week, 2016 October 25 - November 1
|Messier 31, the Great Andromeda Galaxy
Imaged from Morattico, Virginia on 2014 August 30
with an Antares Sentinel 3.1-inch (80mm) f/6 refractor.
The Moon is best seen in the morning hours just before sunrise this week and returns to the early evening sky by week’s end. New Moon occurs on the 30th at 1:38 pm Eastern Daylight Time. You’ll find Luna drifting through the rising stars of early spring, and on the morning of the 28th she will glide just a degree north of bright Jupiter and a degree south of the second magnitude star Porrima in the constellation Virgo. Early next week she will find herself among a crowd of planets and stars as she closes in on Venus and Saturn in the evening twilight.
The absence of the Moon in the evening sky as well as the crisp autumn weather offers us another chance to do a little bit of "citizen science" with the Globe at Night star counting program. This program, sponsored by the National Optical Astronomy Observatory consortium, is designed to map the brightness of the night sky around the world. The premise is simple: find a target constellation during the "dark of the Moon" period each month, compare your view to a simple star chart, and report your results. This month we focus on the constellation of Pegasus, marked by its distinctive "Great Square" asterism, which straddles the meridian at around 10:00 pm. From my yard in the DC suburbs I can usually see the four stars that make up the Square and one or two stars just to the west of it. Skywatchers at much darker locations can see up to a dozen faint stars inside of it!
Last week we mentioned a constellation associated with Pegasus. Andromeda is not only linked by story, but it shares its brightest star with the Flying Horse. Alpheratz is the star that marks the northeast corner of the Square and is the intersection of two chains of stars that represent the chains that bound Andromeda to her fate. The brighter of these two chains curves gently to the northeast and points toward the bright star Mirphak in the constellation of Perseus. This chain is generally visible in suburban skies. A second chain of fainter stars diverges from Alpheratz and follows a more northerly path toward the "W" asterism of Cassiopeia. Just above this chain along about half of its length lies the most distant object you can see with the naked eye. From a dark-sky location it appears as a small detached portion of the Milky Way. In binoculars you’ll see an elongated streak of ghostly light. This is the famous Andromeda Galaxy, the largest member of the Local Group of galaxies. Located some 2.5 million light-years from us, it is about twice as large as or Milky Way galaxy and contains up to 400 billion stars. It was first described by the Persian astronomer Al-Sufi in AD 905 and was one of the first external galaxies to be resolved into individual stars. Visually it is composed of a very subtle diffused light that gradually brightens to a nearly stellar point at its center. It is always one of my highlights for star tours in the autumn sky.
This is the week we observe one of the more interesting astronomical dates in our annual cycle. Halloween is the current way we unwittingly observe in of the "cross-quarter" days that mark the mid-points of the seasons. When we ply the ghosts and goblins that visit us with treats on All Hallows Eve we are continuing a tradition that dates back over 1000 years to Celtic times. We also mark the time of mid-autumn, with winter just a few more weeks away.
Venus continues to climb higher in the evening sky and is the dominant sight in the southwest during evening twilight. This week she overtakes pokey Saturn on the evenings of the 28th through the 30th. At their closest they will be just three degrees apart. The waxing crescent Moon joins them on November 2nd.
Saturn enjoys the visit from Venus, but the bright planet abandons the ringed planet quickly as the week presses on. Saturn’s slow eastward creep is no match for the rapid motion of Venus or the approaching Sun, and Saturn now sets at the end of evening twilight.
Mars keeps pace with the Sun as he moves resolutely eastward toward the stars of the dim autumnal constellation of Capricornus. He’s easy to spot in deep twilight in the southern sky, the brightest ruddy object to be found there.
Jupiter is now becoming a fixture for early morning commuters driving eastward before dawn. Look for a great photo opportunity as the Moon joins Old Jove and the star Porrima in a tight grouping on the morning of the 28th.