Skylights featured on Astronomy Picture of the Day

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Skylights featured five times on Earth Science Picture of the Day:
1 , 2 , 3 , 4 , 5

Sun Pillar

Photo of the Week. A lovely evening sunpillar, caused by reflection of sunlight from ice crystals in clouds, graces the sky above the setting Sun. Compare it with the spectacular morning sunpillar seen in November of 2002.

Astronomy news for the week starting Friday, January 20, 2006.

Skylights now resumes its normal weekly schedule.

The week begins with the Moon in the last stages of its waning gibbous phase, which ends when it passes third quarter the morning of Sunday, January 22. The rest of the week is then spent in the waning crescent phase. The night of Friday the 20th, the Moon will lie to the west of Spica in Virgo, while the following night it will be to the east of the bright star. Then take a look at the Moon the morning of Monday the 23rd as the near-quarter will be found to the southwest of rising Jupiter. The morning of Wednesday the 25th it will be Antares' (in Scorpius) turn to greet the Moon. Finally, the morning of Friday the 27th finds the slimming crescent well to the southwest of Venus. The Moon will actually occult both Spica and Antares, though not as seen from North America.

While Venus is now beginning to make an impact on the morning sky, rising about 6 AM as twilight begins, the morning sky is still dominated by bright Jupiter. Moving easterly against the stars of Libra, the giant planet now quite unmistakably rises around 1:30 AM. The planetary show, however, really belongs to Saturn , which passes opposition to the Sun on Friday the 27th, when it rises at sunset, crosses the meridian at midnight, sets at sunrise, and moves in retrograde as fast as possible (which for distant Saturn is not very fast). Use it as your guide to find the Beehive cluster in Cancer, which lies just north of the ringed planet. A far less visible event (in fact INvisible) is Mercury's superior conjunction with the Sun on Thursday the 26th (when it is more or less lined up with the Sun, but to the far side and about as distant as possible from Earth ). Caught in the middle of all this activity is still-bright Mars. Moving easterly in southern Aries toward the border with Pisces, the red planet now transits the meridian to the south about as evening twilight ends and then sets around 2 AM, just after Jupiter comes up to replace it.

In early evening, watch for Cassiopeia falling into the northwest while the Big Dipper climbs in the northeast, steadfast Polaris always right between them, up from the northern horizon by an angle closely equal to your latitude (the elevation of the North Celestial Pole exactly equal to it, allowing latitude to be quickly found). In the other direction, look for Orion, the seven-starred figure looking much like the outline of the Hunter he represents. Orion is one of three constellations that contain two first magnitude stars (Betelgeuse and Rigel), the others Crux (the Southern Cross, with Acrux and Mimosa) and Centaurus (with Rigil Kentaurus and Hadar). If we adopt Adhara as first magnitude (it lies smack on the border between first and second), we can then also add Canis Major, which (to the southeast of Orion) holds the brightest star of the sky, Sirius, in his paws.

STAR OF THE WEEK: HR 4686 UMI (HR 4686 Ursae Minoris). Eleven naked eye stars (defined as sixth magnitude or brighter, that is, brighter than 6.5) can be found in a six-degree-wide box centered on Polaris, which dominates the set, six in Ursa Minor (including HR 4686, the name from the Bright Star Catalogue), the rest in Cepheus. At sixth magnitude, HR 4686 is the third faintest (beaten out Lambda UMi and HR 286, the latter tucked up against Polaris).
pole Polaris (the jewel in a small semicircle of faint telescopic stars called the "Engagement Ring") is centered in a six-degree-wide field of view that shows a variety of other "polar stars." HR 4686 is just to the right of the top center edge. Lamdba Ursae Minoris is the reddish star up and to the right of Polaris, while Yildun (Delta UMi) is the brighter of the two stars at the upper right corner. Roughly between Lambda and Polaris lies the North Celestial Pole, around which they all seem to revolve.

See the full resolution image, which is linked to a labelled version.
The star's relative faintness comes from a combination of relatively low luminosity (for naked eye stars) and its goodly distance of 152 light years. This class F (F2) star almost reminds us of our own Sun. A hydrogen-fusing dwarf, HR 4686 shines with the light of five Suns from a surface of 7000 Kelvin, not much hotter than that of our own star, from which we derive a radius and mass both 1.5 times solar. But there the relative similarity ends. The slightly higher mass severely truncates the star's dwarf lifetime at 2.7 billion years, less than a third that of the Sun. With an age of 400 million years, "youthful" HR 4686 is still some 15 percent of the way to beginning its death process. The star is distinguished in pair of other ways. Spinning with a minimum equatorial velocity of 68 kilometers per second, 34 times faster than the Sun, its rotation period must be less than 1.1 days. (It is more massive than the divide at class F5 where stars begin to spin faster). It is also a relatively rare "Gamma Doradus star" that varies subtly with multiple periods (0.7 and 4.9 days known), such stars similar to, but just slightly less massive than, the more common Delta Scuti stars. The star's oddest claim to any kind of notoriety is its central position at the bend of the stem of a mostly telescopic "constellation" called "The English Rose," invented by the famed scientist Robert Hooke (1635-1703),
Rose HR 4686 is the bend in the stem of the English Rose, a "constellation" fully visible only through the telescope that was invented by Robert Hooke and presented in his "Discourse on Earthquakes." While the other stars are not visible in the above photo, the charming Rose can be seen down and just to the right of Polaris in the deep image of Ursa Minor. Can you find it?

Information and drawing from Martin Beech, "In Search of the English Rose, Robert Hooke's Lost Constellation," in the Journal of the Royal Astronomical Society of Canada, October 2004, p. 183.
a contemporary of Isaac Newton, thus showing that even the faintest of the sky's naked eye stars can carry their own charm.
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