March 2010 Sky from the Keeble Observatory
Saturn and Mars are putting on a show in the evening – look to the east shortly
after sunset, and you’ll see Mars already above the horizon, looking like a bright
red star. Saturn will rise around 8:00 early in the month, and as much as an hour
before sunset late in the month. These planets represent some of the extremes of
our solar system. Mars is a “terrestrial planet” – think “big rock” – about a third
the size of Earth. Saturn is the second largest of the “gas giant” planets in our
solar system, with its familiar rings stretching a quarter million miles wide, but
only a few tens of meters thick!
Since Mars is relatively close (about 100 million kilometers) it shows a clear disk,
even in a small telescope. Contrast this with the bright point of a star. It can
get closer, but this year’s opposition in late January found Mars approaching its
greatest distance from the Sun (called its aphelion). Its reddish surface is due
to the presence of iron oxide – rust – in the surface rocks and dust. The atmosphere
is very thin, only about 1% as dense as Earth’s, and very cold. Yet, the planet
has fascinated for centuries. Polar ice caps shrink and swell with the seasons.
The highest volcanoes in the solar system are on Mars, the longest rift valley we
know of is on Mars. And, we know that the planet was once much warmer, and wet enough
for rivers to cut channels and floods to rip through the terrain.
Saturn is very different. Its equatorial diameter is about 10 times that of Earth,
yet its diameter measured through the poles is closer to 9 times … the planet is
noticeably “squashed.” This already tells us that it’s not a solid rocky surface
like Mars. Rather, almost the entire planet is gaseous hydrogen and helium. These
are the two lightest elements in the periodic table, and also the most abundant
elements in the universe. Saturn’s composition is almost identical to the Sun –
roughly 90% of all the atoms in the planet are hydrogen, and 9% helium. The rest
of the periodic table accounts for the other 1%. It is flattened at the poles because
it rotates very rapidly, in spite of its size, with one “day” for Saturn taking
a little under 11 hours. The other surprise is its density … at less than 1 gram
per cubic centimeter, Saturn is less dense than water. Given a large enough bucket,
it would float!
Extremes also enter our perception when looking for planets orbiting other stars,
of which we now know of more than 400! (For a current listing of “exoplanets” and
their properties, try the Planetary Society’s web site
http://www.planetary.org/explore/space-topics/exoplanets/.) Our current
methods of finding these planets rely primarily on their gravitational interactions
with the star they orbit. As they move around the star, they “tug” on the star,
causing its apparent position or line of sight velocity (or both) to vary. This
method favors massive planets in close, fast orbits. Not surprising, the exoplanet
catalogs are full of “hot Jupiters” – gas giant planets in close orbits. Another
technique is used by the recently launched Kepler probe, which monitors thousands
of stars looking for a slight dip in their brightness as a planet passes in front
of the star. It is hoped that this probe will find Earth-size planets in Earth-like
orbits. To date, Kepler has found several planets, though no “earths.” One of these
massive planets is even more extreme than Saturn, with a density close to that of
Lunar phases for March: Last Quarter on the 7th, at 10:42 am;
New Moon on the 15th, at 5:01 pm; First Quarter on the 23rd,
at 7:00 am; Full Moon on the 29th at 10:25 pm.
Pre-dawn planet watchers may prefer to sleep in, unless you plan to be up many hours
before sunrise. Saturn sets shortly after sunrise. Jupiter will return to predawn
skies later this month but, it too will be low on the horizon and unlikely observable
through horizon clutter and haze.
Evening planet watchers get a better show. Early in the month look for Mars and
Saturn to the east (see above). By month’s end, Mars will be very high at sunset
(about 65 degrees above the horizon), and Saturn will be rising almost an hour before
sunset, making it viewable almost all night. Mercury and Venus will return to the
western dusk skies. Both will be brilliant, though close to the horizon.
Looking overhead at midmonth, we find Gemini close to zenith, marked by its two
brightest stars, Castor and Pollux. Castor is officially “alpha Geminorum,” which
should mean that it’s the brighter of the two (Pollux is beta Gem), but Pollux is
actually brighter. A modest telescope should resolve Castor as a binary. Mars is
only a bit lower, between Gemini and Cancer. Below Mars, binoculars will make the
Beehive Cluster apparent.
To the east, that bright star above Saturn is Regulus, in the constellation Leo.
With binoculars, a slow sweep just north of the line from Regulus to Saturn will
encounter the elliptical galaxy M 105, about a third of the distance between the
Turning to the northeast, we see the familiar shape of the “big dipper” in Ursa
Major. The middle star in the “handle” is the binary system of Mizar and Alcor.
This pair is resolvable without binoculars or telescope. With a telescope, you can
see that each of these is also binary. Closer to the horizon, at the end of the
handle is Alkaid … and about a degree above and to the right, your binoculars may
enable you to see the spectacular “Whirlpool Galaxy” – also designated M 51.