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Lepus, the Hare

Plus the planets for February

 

Hiding under Orion’s feet is a rabbit attempting to escape from Orion’s Hunting Dogs, Canis Major and Canis Minor. The rabbit, Lepus, the Hare, is a small constellation (51st out of the 88 constellations in size) is composed of eleven stars that form the shape of a rabbit. The brightest star is Arneb (Alpha Leporis), a blue-white star of magnitude 2.6 at the back of the neck of the rabbit. The front of the neck has the second brightest star, the 2.8 magnitude yellow star Nihal (Beta Leporis). To the east and northeast of these two stars is the body of the rabbit and to the west are the head and ears.

star map
Click the map for a bigger chart.

 

Lepus, the Hare, can be found just below Orion in our southern sky. While this constellation does not have brilliant stars of Orion, its two brightest stars, Arneb and Nihal, are both second magnitude. The remaining stars are fainter and it can be difficult to see them all from an urban area. Lepus is just outside the Milky Way, so there are few deep sky objects in this constellation.

While there is no specific Greek mythology with this constellation, it is one of Ptolemy’s classic constellations. The usual representation of Lepus is a rabbit running westward away from Orion’s hunting dogs who are in hot pursuit. One story has Lepus put in the sky by Hermes for his great fecundity and speed, which suits Orion just fine since he was noted for hunting hares. Another legend has the early colonists of the island Leros bringing hares to the island. They multiplied quickly and overran the island. The people of Leros eventually drove them out and they put Lepus in the sky to remind everyone of the benefits of moderation. Among the stars that make up Lepus is Gamma Leporis (a.k.a. Gamma Lep) in the southeast part of this constellation. To the naked eye, this appears as a white star shining at magnitude 3.6. This is a spectral class F6 star, so it is a little larger and hotter than our Sun. Gamma is 23% more massive than our Sun and it is 33% larger in diameter. It is also a nearby neighbor of ours, only 29.3 light-years away. If you hold up a finger at arm’s length, and move your head back and forth, the finger will appear to move relative to the more distant background. Astronomers do the same thing with a star, but instead of moving their head from side to side, they let the Earth move from one side of the Sun to the other. In other words, they measure the position of a star against the background stars and then wait for the Earth to travel half way around the Sun, which takes half a year. They then measure the star against the same background and measure its shift. Then using trigonometry, they can directly compute the distance to the star.

Because it is hard to get precise measurements looking through the Earth’s blurring atmosphere, astronomers launched the Hipparcos satellite into space to make these measurements without the blurring effects of the atmosphere.

From measurements of Gamma Leporis, we know this star is 29.3 light-years away. If you look at Gamma with a pair of binoculars, you will that Gamma has two other stars nearby. Gamma’s nearest companion is a magnitude 6.2 orange star of spectral type K2. It is also 29.3 light-years away, so it must be actually related to Gamma, making these two a binary star. The third star is much further away, so it is not physically related to Gamma.

These three stars make up a triple star. Since only two of these stars are physically related, astronomers call Gamma an optical triple star. In optical doubles, triples, etc., the stars appear along the same line-of-sight, but may not be related. The term binary star on the other hand always means they are physically related.

Knowing this, the bright star we have been calling Gamma Lepus should be referred to as Gamma Lepus A, while the fainter star would be Gamma Lepus B. If the third star were related, it would have been Gamma Lepus C. Astronomers now know that the distance between Gamma Lep A and Gamma Lep B is about 870 times the distance between the Earth and the Sun. This means it takes 18,000 years for the two to go around each other. At this distance, someone on a planet near Gamma Lep A would see B shining as bright as a quarter moon, while someone on a planet near Gamma Lep B would see A shining as bright as a full moon.

The Planets for February 2015.

Venus dominates the evening sky as it gets dark. It climbs higher in the sky each night, being almost twenty degrees up in the west-southwest as it gets dark. At that time, Venus’s disc will be 11.5 seconds-of arc across and it is 89 percent illuminated and decreasing. Shining at magnitude -4.0, it sets around 8 p.m. During the month, the Goddess of Love travels from central Aquarius into Pisces, clipping a corner of Cetus and back into Pisces by month’s end.

Venus is not alone in the low western sky. On Feb. 21, the eastbound Venus will pass half a degree south of the slower moving Mars. Mars’s disc will be 4.5 seconds-ofarc across and it sets around 8:15 p.m., shining at magnitude +1.3. During the month, the God of War moves eastward from eastern Aquarius into central Pisces.

Jupiter is in opposition on Feb. 6, when it will be on the opposite side of the Earth from the Sun. This is when Jupiter is closest to the Earth, making it the best time to observe the King of the Gods. It will be moving slowly westward from western Leo into eastern Cancer. It rises in the east-northeast as it gets dark and is visible the rest of the night. Jupiter shines at magnitude -2.6 and its disc will be 45.3 seconds-of-arc across at mid- month.

Watch the Skies
All times MST


Feb 3:  4:09 p.m. Full Moon
Feb 6:  11 a.m. Jupiter opposite side of Earth from the Sun
Feb 11:  Midnight, Mercury stationary
               8:50 p.m. Last Quarter Moon
Feb 18:  4:47 p.m. New Moon
Feb 20:  6 p.m. Mars 1.5° S of Moon
Feb 21:  1 p.m. Venus 0.5° S of Mars
Feb 24:  9 a.m. Mercury greatest distance W. of Sun (27°)
Feb 25:  10:14 a.m. First Quarter Moon

Shining at magnitude +0.5, Saturn rises around 1:30 a.m. in the east-southeast. The Ringed Planet’s disc is 16.5 seconds-of-arc across with its Rings 37.4 seconds-of-arc across. They are tilted down 25.0 degrees with the northern face showing. During the month, Saturn moves very slowly eastward in the western half of the panhandle of Scorpius. After last month’s adventure in the evening sky, Mercury makes an appearance in the morning sky this month. Emerging from the Sun’s glare after the first week of the month, it reaches its greatest distance from the Sun on the February 24. On that day it rises at 5:20 a.m. with a disc that is 7.0 secondsof- arc across that will be 55 percent illuminated and increasing. The Messenger of the Gods will shine at magnitude +0.1.

It moves westward in western Capricornus during the month, almost reaching the border with Sagittarius before it turns back eastward ending the month in central Capricornus. This will not be a great apparition with Mercury only nine degrees above the horizon as it gets dark, but you can still “keep watching the sky!”

 

An amateur astronomer for more than 45 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.





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