It's always a mixed bag for astronomers. Spring just arrived in the northern hemisphere, and the temperatures are less frostbitey than they were a few weeks ago. On the other hand, the nights are growing shorter. Oh, it still gets dark fairly early, and a person can get in quite a bit of viewing without staying up late.
This week, the planets are the main event, and sunset is the time to see
them. All five naked-eye planets are visible in the early evening. Even early-to-bed
types can tour the solar system without losing sleep.
The four brightest planets - Venus, Mars, Jupiter, and Saturn, have been
visible for some time now. For a couple of weeks, Mercury will also be visible
in the dusk glow.
Mercury is the planet closest to the sun. In some ways it resembles our own
moon - similar in size, and a target for celestial debris because it lacks
an atmosphere. This has another surprising effect. Mercury is less than half
the distance of the earth to the sun. Predictably, this makes for some mighty
hot days, up to 740 degrees Fahrenheit. Without the stabilizing influence
of an atmosphere, though, nights are shockingly cold, with temperatures plunging
past -300 degrees.
Some of those days and nights are long, too. Mercury orbits the Sun in 88
days, and rotates on its axis every 59 days. The result is that the sun can
be illuminate the same point on Mercury's surface for up to three months
at a time, tracing a ragged path back and forth across the sky.
Because it is so close to the sun, Mercury is always less than 30 degrees
away from the sun as we observe it. Much of that time it is lost in the sun's
glare. When Mercury is visible, it is only for a few weeks at a time. It
stays close to the horizon in early evening or early morning twilight. An
unobstructed horizon is vital for viewing the planet.
Mercury is rising higher in the sky now, and will reach its highest point
on March 29. Then, it will descend back into the sun's glow over the next
two weeks. On March 21 and 22, the one- and two-day old moon will be nearby
in the very early in the evening. On the 22nd, look for Mercury to the right
and below the moon.
During the week, the waxing crescent moon will also visit bright Venus on
the 24th. Venus is also nearing its highest point in the sky for this season.
March 25, the moon will visit Mars. Mars and Venus are on either side of
the Pleiades, making an excellent target for astrophotographers. Saturn,
high overhead at sunset, and Jupiter, prominent in the west and visible most
of the night, round out the planetary show.
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