Constellation of the Month: Cetus, The Whale
Last January "The Starry Dome" told the story of Andromeda and the trouble her mother caused by bragging that her daughter was prettier than Nereids, a sea nymph. This upset Nereids, who went to Neptune to get revenge for the comment. Being a sea god, Neptune sent a sea monster to pillage the kingdom's coastline. Whether Cetus represents a sea monster or, as he is depicted today, a whale, he wreaked havoc on the coast. The king was given a prophesy that he should chain Princess Andromeda to the rocks as an offering to the sea monster and to Neptune. To save his country, he did so.
Cetus was about to chomp on Andromeda, when Perseus, having just dispatched the gorgon Medusa, spied Andromeda and fell in love with her. Eventually, he dispatched Cetus, either by turning him to stone with Medusa's head, or slicing in-between the monster's armored scales to reach a vital spot. With Cetus' death Andromeda and Perseus were free to go on their merry way. Neptune asked Zeus to put Cetus in the sky. Zeus did so, but he was careful to put Cetus far from all the other sky-resident characters in this myth.
Cetus was around long before the Greeks, showing up in the writings of other ancient cultures. The Roman historians Pliney and Solinus recorded that the skeleton of Cetus was 40 feet long and the circumference of the vertebrae was six feet. This skeleton was so interesting that Scaurus brought the bones back to Rome as a public spectacle.
The most interesting object in Cetus is the star Omicron Ceti. In 1596, while searching for Mercury, the German astronomer David Fabricius discovered that this was a variable star. It was later determined that Omicron Ceti varied in brightness from third magnitude, which is easily visible, all the way down to ninth magnitude, making it invisible to the eye. The cycle takes 331 days, making this the first long-period variable star to be discovered. This star was name Mira, meaning "wonderful," in 1642 by Johannes Hevelius of Danzig. There is a whole family of variable stars with large brightness changes and long periods know as Mira-type variable stars.
Mira itself is a cool red giant star some 5,400 trillion miles away, which has been recently discovered to be part of a binary system. Mira is classified as a pulsing star; it expands and contracts with a regular period. The outward pressure from inside the star overwhelms the pull of gravity, the outer atmosphere of the star expands outward and the star gets brighter. Eventually, the increasing volume reduces the pressure and gravity takes over again to pull the outer atmosphere back close to the star. Then the process starts all over again. For some stars, like Mira, this happens on a regular basis, while other pulsing stars do not keep to any schedule.
The Planets for December 2005
The brilliant gem in our southwestern sky just after sunset is the planet Venus. Crossing from Sagittarius into Capricornus around midmonth, Venus is slowing its eastward progression among the stars and will turn back toward the Sun early next month. The peppy Venus is preparing to slip between the Earth and the Sun. When it does so, it will become an even narrower crescent. On Dec. 15, Venus will be a 20-percent-sunlit crescent some 46.5 seconds-of-arc across and will glow at magnitude -4.7. Venus will be at its brightest for this apparition on Dec. 9.
The ruddy-hued Mars is in the eastern sky at sunset this month. It has passed its closest point to us, and we are now pulling away from Mars, making it appear smaller and fainter. It is still a good target for a telescope, 14.3 seconds-of-arc across at midmonth. This magnitude -1.1 planet will be in the constellation Aries all month. There is a Martian dust storm in progress, and this may obscure the view of the planet's surface.
The Ringed Planet, Saturn, comes up around 9 p.m., and is visible for the rest of the night. Remaining in Cancer, Saturn continues to show off the southern face of its rings, which are now 44.7 seconds of arc across. The ball of the planet is 19.7 seconds of arc across. Saturn is slowly moving nearer the Earth, so it will continue growing in size and brightness.
Jupiter is in the morning sky, slowly pulling away from the Sun after its conjunction at the end of October. Rising around 4 a.m., the Giant Planet is becoming better placed for telescopic observing. Midmonth will see Jupiter glowing at magnitude -1.8 and stretching 32.2 seconds of arc across.
Mercury will pop up in the morning sky, being farthest from the Sun (21 degrees) on Dec. 12. It will be magnitude -0.4 and 6.6 seconds of arc across. Mercury will become visible in the first week of the month and will remain visible into early 2006. Telescopically, Mercury is about 62 percent illuminated, a gibbous planet that will become "fuller" as the month progresses.
Northern Hemisphere winter begins on Dec. 21, at 11:35 a.m., when the Sun reaches its southernmost excursion for this year. Also, the Moon will hide the bright star Spica on Christmas morning around 6:24 a.m., for those who live east and north of Las Cruces. Those south and west will just see the star skim along the southern edge of the Moon. For those who will be graced by this spectacular occultation, consider this an early Christmas-morning present and "keep watching the sky!"