"Jeanius at werk."
THE DAILY ASTRONOMER
Monday, September 19, 2016
Night Sky Calendar: Week 3
September 19 - 25, 2016
We do hope these weekly sky calendars have proven quite helpful. We switched from the monthly to weekly format so as to provide more detail for each entry. Please let us know your thoughts. A subscriber did ask us to bring back the "Planet Watch" section, which returns today The Planet Watch will still be monthly, but we'll include it at the end of each weekly sky calendar. Also, at the subscriber's behest, we'll continue to select a pick planet.
WEDNESDAY, SEPTEMBER 21, 2016: MOON 9.2 DEGREES SOUTH OF THE PLEIADES
The waning gibbous moon, being only 70% illuminated tonight, will not wholly obscure the Pleiades Star Cluster, the open star cluster just to the west of Taurus the Bull. This event afford us the opportunity to explore our next Messier object. (Recall that this school year we're going to "visit" every Messier object. This is our way to pay tribute to Charles Messier on the 200th anniversary of his death - April 12, 1817)
Pleaides Star Cluster. "The Seven Sisters." An open starcluster within Taurus the Bull. Designated M45, this cluster
is one of the few naked eye Messier objects. To the unaided eye,
it appears like a faint cloud. Viewing it through a telescope
or even binoculars reveals its richness. IMAGE: Hubble Space
Telescope
The Pleiades is also known as the "Seven Sisters," because an observer can observe about seven individual stars. Through a telescope, one can count a few hundred member stars. Approximately one thousand stars have been shown to be members of a cluster, the core of which extends over eight light years. This cluster formed about 120 million years ago and, according to computer simulations, will slowly dissipate over 250 million more years. At that time, most of the component stars will have established their own trajectories through the galaxy. In the above photo, we can observe rarefied gases enveloping the stars. These gases comprise the "Maia Nebula," so named as it was thought to have been the remnant of the nebula from which these stars took form. Astronomers have since determined that this nebula is not associated with the cluster's birth. Instead, the Pleiades Star Cluster is currently passing through the cloud. The starlight illuminates the cloud, producing a glow reminiscent of the luster fireflies impart onto a fog bank.
As we proceed through autumn and then winter, the Pleiades will rise earlier each night and will become ever more prominent.
WEDNESDAY, SEPTEMBER 21: MERCURY STATIONARY
If you were an astronomer and charged with tracking the planets, you would need to observe the planets every clear night and note their positions relative to the stars. You would notice that the planets tend to move eastward relative to the stars. At times, however, this eastward migration would stop and then the planet would move westward (retrograde) against the stars. Eventually the planet would appear to stop again before resuming prograde motion. When the planet reaches one of these "stopping" points, it is said to be 'stationary.' A planet never actually halts in its orbit. Instead, it appears to do so because we are moving relative to the planet. Mercury is an inferior planet, meaning that it is closer to the Sun than Earth. We're watching this little world move around its orbit from an external perspective. When Mercury reaches either "end point" on this orbit, it appears stationary. Similarly, a car driving around a track will either appear to move to our left or our right depending on which side of the track it occupies.
THURSDAY, SEPTEMBER 22, 2016: AUTUMNAL EQUINOX!
We're passing from one astronomical season to the next. We have seasons because Earth is tilted on its axis by 23.5 degrees, a value known as the "obliquity." Our summer solstice occurs when Earth's northern hemisphere is aligned toward the Sun as much as possible. When Earth's southern hemisphere is poised toward the Sun, our winter begins. At either equinox, neither pole is aligned toward the Sun more than the other. The autumnal equinox occurs after summer, but before winter. Interesting note: Earth reaches aphelion -its greatest distance from the Sun- in early July, so our planet is farthest from its parent body in our summer. Consequently, it's traveling at its slowest orbital speed between the summer solstice and autumnal equinox. Therefore, in the northern hemisphere, summer is the longest season. Of course, it still has to come crashing to an end. Autumn begins with a whimper at 10:21 a.m. EDT!
FRIDAY, SEPTEMBER 23, 2016: LAST QUARTER MOON
PLANET WATCH
MERCURY: becomes visible in the pre-dawn sky by the third week of September. It is elusive, as always, but by the end of September grows brighter than Saturn and Mars. VERDICT: Wait until the very end of September to see Mercury in the early morning eastern sky.
VENUS (PICK PLANET!) Venus maintains a constant brightness throughout September, but, of course, remains brighter than all the other planets and all the night sky stars. Even though Venus sets during the very last twilight phase this month, it is still a brilliant western sky object and easy to find if you have a low western horizon.
VERDICT: If you're out at dusk, seek out Venus all month!
MARS: Mars had its time in the limelight in May when it reached opposition. Although it remains in the western evening sky for the rest of the year, it will diminish sharply in brightness through autumn. VERDICT: See Mars in the southwestern early evening sky each night this month. As autumn ages, Mars will grow dimmer..
JUPITER: Not visible this month! This bloated gas giant returns to the pre-dawn eastern sky by mid October. VERDICT: Well.....please wait until late next month.
SATURN: Is dimmer than Venus, Mars and, by late September, will also be dimmer than Mercury. This planet will vanish into the dusk by November. Now, it rises about four minutes earlier each night. VERDICT: Find Saturn in the early evening this month. It will become much harder to find in October as it approaches its hiatus.