THE SOUTHWORTH PLANETARIUM
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70 Falmouth Street Portland, Maine 04103
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Founded January 1970
Julian date: 2457673.16
"Dancing with the stars."
THE DAILY ASTRONOMER
Monday, October 10, 2016
Week 6 Night Sky Calendar: October 10 - 16, 2016
Well, of course time is elapsing quickly. The Universe has trillions of years of time left in its stores and is rapidly trying to distribute it all. This week, the remote and distant gas giants Uranus and Neptune put in appearances. We also see a pleasant pre-dawn planetary appulse and a bright super moon! While we point out only the most interesting events (interesting to us) thousands of other events will transpire this week that will go unnoticed. One consequence of living in such a dynamic Universe is that so much is happening in so many places that we have to pick and choose only a few out of a myriad.
TUESDAY, OCTOBER 11: MERCURY 0.79o NNE OF JUPITER (BRONZE EVENT!)
Last week we mentioned that Jupiter would reappear around the same time Mercury vanishes. This morning, one will be able to observe both planets low in the eastern pre-dawn sky. Although these worlds will appear to be separated by less than one degree, they are actually more than 480 million miles apart. (The night sky is very good at concealing depth.) Jupiter will be about 1.7 times brighter than Mercury, enabling one to easily distinguish between them. Unfortunately, both planets will rise about an hour before sunrise. As these planets are quite bright (Mercury is almost as bright as Sirius; Jupiter is slightly brighter than Sirius), one can see them both for about 45 minutes after they appear. Even though planetary appulses are quite spectacular, this event received only a bronze because the two planets will be so low in the eastern sky.
THURSDAY, OCTOBER 13: MOON 1.1o NNW OF NEPTUNE
Neptune is not a naked eye planet, but this week the pickings are slim and in desperation we decided to include this event. When you observe the waxing gibbous moon tonight, realize that the eighth planet is hidden within the darkness just below it. Astronomically, this event is interesting because it will actually be a lunar occultation visible from the extreme northern regions of Earth. During a lunar occultation, the moon actually moves in front of a more distant object. The moon subtends a 1/2 degree angle and moves its own diameter every hour, and so occults stars constantly. However, it will occasionally occult a planet or one of four first magnitude stars (Aldebaran, Regulus, Spica or Antares)
SATURDAY, OCTOBER 15: URANUS AT OPPOSITION
Yes, one can see Uranus without a telescope. However, it is quite faint. An observer must be in a completely dark sky and also know Uranus' precise location in order to find it. Even if observed, the seventh planet will merely look like a faint star. Uranus is at opposition today, meaning that Earth will pass between this planet and the Sun. Planets at or around opposition are at their maximum brightness. Even though Uranus is at its maximum brightness for 2016, it is still only about 1.4 times brighter than the faintest naked eye stars. One can find Uranus within the constellation Piseces the Fish. Refer to the star chart below.
Uranus at Opposition Uranus is actually visible to the unaided eye…just. One
must observe the sky from a dark location and precisely know Uranus' location relative
to the background stars. The chart above (produced by iomastronomy.org) pinpoints
the seventh planet's position against the stars of Pisces the Fish.
SUNDAY, OCTOBER 16, 2016 FULL MOON (A SUPER MOON!)
We've bought into the "super moon" hype because it is so much fun and, like all introverted astronomy types who experience heart palpatations whenever falling acorns crack twigs, we're incorrigible excitement-addicts. This month's full moon is a SUPER MOON! A super moon is merely a full moon that occurs at or near perigee, the point in a moon's orbit that brings it closest to Earth. Such moons don't occur every month, but they aren't extremely rare, either.
To explain why we don't have supermoons every month, we need to actually discuss two definitions of "month." Our calendar month is based on the moon's "synodic period," the time period separating successive new moons. This "synodic month" is about 29.5 days, give or take a bit. A phase cycle requires about 29.5 days for completion.
The second type of month is called "anomalistic." Equalling 27.55 days, an anomalistic month is the time period separating successive perigees. As we mentioned, "perigee" is the closest point between Earth and the moon during any given orbit. The moon travels along an elliptical orbit and so its distance from Earth veers between two maxima: perigee and apogee, the point of greatest distance. The anomalistic and synodic periods differ by about two days and so are out of sync. Most of the time, when the moon is full is is not close to perigee. When the moon is full around the time of perigee,we have a super moon!
"Perigee and apogee" points. This graphic shows the difference between perigee (point of least distance between Earth and the moon) and apogee (the point of greatest distance.) The moon travels along a slightly elongated ellipse, so its distance from Earth continually changes. The moon reaches its maximum distance from Earth at apogee and its least distance at perigee.
This month, perigee occurs 19.4 hours after the full moon, giving us a Supermoon! The actual term is a "perigee-syzygy event."* A super moon is about 14% larger (by diameter) and 30% brighter than an apogee full moon. However, one would be hard pressed to tell the difference between an apogee full moon and perigee full moon, as one cannot observe them simultaneously.
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SPACE-TIME SNEAK PEEK:
November's Super moon (11/14) is going to be hyper-hyped! Perigee will happen 2.4 hours before the moon is full and it will be the closest perigee in 30 years. Also, the election will have been over for less than a week and we'll all be clamoring desperately for something to entertain us.
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PLANET WATCH
MERCURY (Host constellation: Virgo)
Mercury is only visible early in October and then vanishes for the rest of the month. Even when visible, Mercury is low in the pre-dawn eastern sky. Mercury passes into superior solar conjunction on October 27th and won't return to the evening sky until late November. VERDICT: Even by Mercury's standards, it is not an easy planet to find this month. If you must try, do so at the very beginning of October. Otherwise, wait until around Thanksgiving. Yes, it will be that long of a wait. This week, say goodbye to Mercury on the morning of October 11, when it comes within less than a degree of Jupiter.
VENUS (Host constellation: Libra/Scorpius) PICK PLANET!
Venus gets the crown again because it is so bright, even if it doesn't remain in the western evening sky for long. Venus brightens only slightly throughout the month, but, as always, is brighter than any night sky star. VERDICT: It will be easy to find Venus provided you venture outside in the early evening. Look to the west and see the brilliant Venus. Best day to look this week will be October 3, when Venus will be close to the moon
MARS (Host constellation: Sagittarius)
Though it is not as bright as Venus, Mars is easy to find in the western evening sky. Interesting note: Mars will be setting around 9:30 p.m. each night throughout the month. This equivalence occurs because of the way Earth and Mars are moving relative to each other at the moment. VERDICT: Seek out the red world in the evening this month. It will remain above the horizon for a couple hours after sunset.
JUPITER (Host constellation: Virgo)
After a month-long hiatus, Jupiter retrurns to the early morning eastern sky. Consider this to be a perfect "changing of the guard." As Mercury vanishes in the pre-dawn twilight, Jupiter emerges. Though not as bright as Venus, Jupiter shines brighter than any night sky star. VERDICT: If you've dearly missed Jupiter, just venture outside to find it starting mid-month. Jupiter will brighten throughout the rest of 2016 as it rises earlier each morning. This week, seek out Jupiter on the morning of October 11, when it comes within less than a degree of Mercury.
SATURN (Host constellation: Libra)
We'll lose Saturn next month. In October, Saturn lurks low in the southwestern evening sky. It is still a moderately bright planet and will be easy to find, at least early in October. VERDICT: Seek out Saturn at month's beginning just after dark. Still a beautiful world, particularly through a telescope. Although we lose Saturn in November, we'll regain it in late December.
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If we want to reach 100 by or before August 4, we should reach into the vessel more often.
PANDORA PARCHMENT # 3: "Why can Mercury never be farther than 28 degrees from the Sun in our sky? Also, does that change if you are at a different lattitude?"
-P.H. , Florida
Mercury can never appear more than 28 degrees from the Sun in our sky because it is so much closer to the Sun than Earth. As the diagram below illustrates, Mercury reaches this maximum elongation only when it is in a specific part of its orbit. If Mercury were even closer to the Sun, its maximum elongation angle would be even lower!
Mercury's Maximum Elongation. Mercury can never appear more than 28 degrees from
the Sun in our sky because it is much closer to the Sun than Earth. This angle is independent of latitude.
This image -which certainly isn't to scale- is courtesy SWOSU.
This maximum elongation is independent of latitude. However, Mercury's angle in the sky does depend on the latitude. If Mercury is south of the ecliptic, its apparition will be more favorable for southern observers. However, its solar elongation will be the same for all observers.
Side note: Earth and the other inner planets are generally well hidden from the perspective of outer planets. An astronomer on Saturn, for instance, would probably never see Earth, which wouldn’t appear to stray more than 7 degrees from the Sun! Well, a Saturnian astronomer could observe Earth when it transits (moves in front of) the Sun. The next transit of Earth from Saturn occurs on July 20, 2020.
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*The term "super moon" was coined by an astrologer, so we astronomy types are supposed to be loath to use the term. Our aversion to astrology is a cherished tradition. Considering the relative marketability of astrology and astronomy, borrowing money from astrologers is another one.