Saturday: Is your favorite astronomy-loving relative asking for a telescope this Christmas? Before reaching for your credit card, read this guide to choosing your first telescope, available at http://goo.gl/5oXmGj. If cost is an issue, look no further than this article about low cost telescopes https://goo.gl/8yyddy. These are not cheap telescopes. They are simple, low-cost, easy to use telescopes that your future astronomer will still use for quick observing sessions long after she has purchased a much larger instrument for richer viewing. If you want to give a gag astronomy gift to someone who really bugs you, give them a copy of this column. After such a dud “gift”, you’ll never hear from them again. And that may be the best gift of all.
Sunday: Lieutenant Worf, the Klingon Starfleet officer on
Star Trek: The Next Generation, might say “Today is a good day to die.” But
Deneb, the bright supergiant star in Cygnus the Swan would say “two million
years from now is a good day to die.” This may seem like a long time. But,
compared to the lifespan of most stars, two million years from now is as close
as tomorrow. For example, the Sun will last about five billion years. Small
stars known as red dwarfs may last trillions of years. Prepare your
astronomically short goodbyes to Deneb tonight at 11:00 o’clock when it is two
fists above due northwest.
Monday: At 5:00 p.m., Jupiter is about three and a half fists above the southeastern horizon. Saturn is two and a half fists above the southern horizon.
Tuesday: It’s getting too cold to see frogs in the wild. But
this is a great time to see frogs in the sky. Ancient Arabs referred to the
stars that we now call Fomalhaut and Diphda as Ad-difdi al-awwal and Ad-difda
at-tani. This means the first frog and the second frog, respectively. Both
frogs are low in the southern sky at 5:30 p.m. Fomalhaut is nearly one and a
half fists above due south. The slightly dimmer Diphda is two fists above the
south-southeast horizon.
Wednesday: Tonight’s December full moon is called the Cold
Moon… for obvious reasons. On the evening of a full moon, the moon rises as the
Sun sets, reaches its highest point in the sky in the south at midnight, and
sets as the Sun rises. Mars is in opposition tonight. When a planet is in
opposition, it rises as the Sun sets, reaches its highest point in the sky in
the south at midnight, and sets as the Sun rises. Thus, the moon and Mars move
across the sky together… almost. Since the moon is so much closer to the Earth
than Mars, its own motion affects the path we see. As the moon and Mars appear
to move from the eastern to the western horizon due to the Earth’s rotation,
the moon moves slightly eastward compared to Mars, due to the moon’s own orbit
around Earth. At about 6:50 p.m., the moon passes between Mars and the Earth,
occulting Mars. Mars disappears behind the left side of the moon. About an hour
later, Mars appears from the right side of the moon.
Thursday: The earliest sunset of the year in Ellensburg
occurs this weekend: 4:12 p.m. This seems odd because the shortest day of the
year, the winter solstice, isn’t for about two more weeks. The Sun is at its
southernmost point with respect to the background stars on the day of the winter
solstice. This means the Sun spends the least amount of time above the horizon
on that day. But, the sunrise and sunset times depend on more than the Sun’s
apparent southward motion in the sky. It also depends on where the Sun is on
the analemma, that skinny figure-8 you see on globes and world maps. During the
second week in December, the Sun is not quite to the bottom of the analemma.
But, it is on the leading edge of the analemma, the first section to go below
the horizon. For a slightly better explanation of this, including a diagram, go
to https://go.shr.lc/2NOMOQC. Or just
go watch the sunset. But don’t stare at the Sun.
Friday: Mercury and Venus set just after the Sun so they are
a challenge to see. Look just above the southwestern horizon right after the
Sun has set. Venus is the brighter point of light. Mercury is just to the left
of it. If you don’t find them tonight, that’s okay. They’ll be moving farther
away from the Sun in the sky, making them easier to spot by mid-month.
The positional information in this column about stars and
planets is typically accurate for the entire week. For up to date information
about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.
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