Skylights featured on Astronomy Picture of the Day

Scout Report Selection Webivore Selection SpaceCareers Selection

Skylights featured nine times on Earth Science Picture of the Day: 1 2 3 4 5 6 7 8 9 -- Full List Restored!


Photo of the Week.. Mars rising.

Astronomy news for the two weeks starting Friday, December 2, 2016.

The next skylights will appear December 16, 2016.

We span much of the waxing lunar phases this fortnight, the Moon going from thin waxing crescent as we open our session to first quarter on Wednesday, December 7, then through waxing gibbous, which ends at full Moon on Tuesday the 13th. We then see a piece of waning gibbous to bring us to the end.

Four of the ancient planets will appear in western evening twilight. The slim crescent will be glorious to the right of Venus in western twilight the evening of Friday the 2nd, then will bear about the same relation with Mars two nights later. On Saturday the 3rd, the crescent will appear on top of the brilliant planet down and to the right of the red one, the changing aspects a treat to watch. Mercury then gets into the act with greatest eastern elongation taking place on Saturday the 10th. Mars reliably sets at 9:30 PM. Later in the evening, Jupiter has risen by 3 AM, while Saturn invisibly passes conjunction with the Sun on Saturday the 10th.

Back to the Moon. On the night of December 12-13, the near-full Moon will occult Aldebaran in Taurus for the whole lower US. The time of the event depends on location, and runs between 7 pm and after midnight. Watch during the late evening hours. However it's tough to see without at least binoculars.

The Moon passes perigee, where it is closest to the Earth on Monday the 12th.

One of the blanker areas of the sky lies between the North Celestial Pole and the line that connects Auriga and Perseus. Where better to hide Camelopardalis, the Giraffe, one of the 38 modern constellations that were invented roughly between 1600 and 1800 to fill in the areas with few bright stars, Camelopardalis extending almost the the Pole itself.

STAR OF THE WEEK: 1 CAM (1 Camelopardalis). "Number 1" makes the star seem impressive, Unfortunately, the brightest star in dim Camelopardalis (the Giraffe), Beta Cam, is only fourth magnitude, while 1 Cam shines at but fifth magnitude (5.43). "Number 1" is actually a Flamsteed number that refers to the westernmost modestly bright star within the constellation, not brightness itself. Dim the Giraffe might be, it holds a plethora of mighty stars, 1 Cam easily included among their number. A first look reveals it to be (from Smythe and Chambers) "A neat double....nearly in mid- distance between Alpha Persei and Delta Aurigae, A 7 1/2 white, B 8 1/2 sapphire blue." In reality they both are blue-white, a sixth magnitude (5.78) Class B (B0) giant coupled to a seventh magnitude 6.82 class B0 subgiant- dwarf separated by 10.4 seconds of arc, making it an easy telescopic object. The problem is that we don't know how far away it is. The star is too far for parallax, and because of the inevitable errors, the parallax actually comes out negative. All we can do is to get the visual magnitudes from the spectral classes as derived from other stars in the HR diagram of magnitude vs. class (respectively -4.5 and -4.0)and compare them to the observed magnitudes after correction for dimming by interstellar dust (a whopping 1.52 magnitudes). We then get 2970 and 3830 light years for 1 Cam A and B for an average of 3400 light years. In support, 1 Cam by odd coincidence belongs to the Cam OB I association of hot blue massive stars ("associations" groups of stars formed out of the same molecular clouds but unlike clusters are gravitationally unbound, such that the members gradually separate). Broad studies of Cam OB I place it 3300 light years away: not a bad fit! The temperatures are essentially unknown. From the spectral class we'll adopt 27,000 K, which gives the amount of ultraviolet radiation to be added to the visual and thus gives luminosities of 55,000 and 35,000 Suns, radii of 10.7 and 8.5 times that of the Sun, and masses of 18 and 16 Suns, almost assuring that both will blow up as supernovae in the not-too-distant future, especially since they are too far apart (at least 10,800 Astronomical Units) to affect each other during supergiant evolution. But are they really a pair? Over the past two centuries they have changed their separation by a second of arc. Given their physical separation, which from Kepler's Laws gives an orbital period of at least 190,000 years, they may not be bound at all, just related through their membership in the association and like all the other hot stars are slowly drifting apart. A twelfth magnitude star hovers in the area 150 seconds from the inner pair, but it is probably just in the line of sight. Oddly, 1 Cam A is spinning with a projected equatorial velocity of 280 kilometers per second (giving it a rotational period of under two days), whereas 1 Cam B is hardly rotating at all, the velocity too low to be measured. Finally 1 Cam A had the reputation of being a rapidly chattering Beta Cephei-type variable. Alas, the distinction has been withdrawn. If there is a moral, it's don't bet the quality of the stars on the basis of the dimness of constellation.

Valid HTML 4.0! Copyright © James B. Kaler, all rights reserved. The written contents and (unless otherwise specified) the photograph are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes.