December 2008 Sky from the Keeble Observatory
Some quick updates: The Hubble Space Telescope has resumed normal science
operations, with all instruments (the ones that still work!) switched to “Side B”
Mars Phoenix is no longer communicating due to insufficient electrical power.
It’s unlikely it will survive the Martian winter … it’s expected to be coated with
a layer of frozen carbon dioxide by the time spring rolls around in six months!
We’ve been looking at how astronomers use basic geometry and trigonometry to measure
distances in the solar system. Now we can look beyond, to the stars.
Try this, first. Hold a finger at arm’s length and look beyond it to a more distant
object – look first with your left eye, then your right. Now, move the pencil closer
and repeat. The effect that you will notice, the apparent shift in position of the
nearby object as you blink from eye to eye is called parallax. It’s the basis
of stereo vision and depth perception, which is why you get checked for this when
renewing your driver’s license. Knowing the distance between your eyes, and measuring
the angles towards the background from each eye would allow you to draw a triangle
and measure the distance.
This is the basis of measuring stellar distance using parallax, but we don’t rely
on the few inches between your eyes. Rather, we measure directions to stars from
opposite sides of the Earth’s orbit around the Sun. A relatively nearby star will
shift its apparent position against the more distant “fixed stars” over the course
of a year. The shift over any given six month period will yield the maximum change
in position. A diameter of the Earth's orbit forms the base of a triangle, with
the star at the apex. The angle at the star is the angular shift in position over
that six month interval.
Now we’re back to triangles! If we divide the six month angular shift in half, we
define that angle as the parallax, usually measured in seconds of angle. (One second,
written as 1” is 1/3600 of a degree.) We further define the distance at which the
parallax is 1” as a parsec. A little simple arithmetic will tell us that
1 parsec is 206,265 times the distance from Earth to Sun … about 3.26 light years!
One argument long used to claim that the Earth was not moving was the lack of apparent
parallax – but the reason you don’t see it is because the angles are so very small
that you cannot measure them without a telescope and some precision instrumentation
for comparing stellar positions. The first successful parallax measurements were
done in the 19th century, and we came to realize that the nearest stars
are more than a parsec away! The closest Sun-like star, Alpha Centauri lies
at a distance of just about 4 light years. That’s 1.23 parsecs, which means that
its parallax is less than 1”!
For practical reasons, the limit of directly measuring parallax from ground based
telescopes is about .01”, which means that we can only get direct distance measurements
within a radius of 100 parsecs, or about 235 light years. To improve on those distances
but still apply the basic technique, we rely on data from the Hipparcos satellite,
launched in 1989 and which completed its mission in the mid 1990s. These data extended
the parallax limit by nearly an order of magnitude to approximately .002”, and allowed
distance calculations for a sphere out to about 500 parsecs. Even though that sounds
like a huge distance, it’s really just our nearest neighborhood in the larger scheme
of things. For example, the center of the Milky Way Galaxy is 6500 parsecs from
To go further, we’ll need to study in detail those stars within our ability to directly
use parallax, and try to extrapolate to the next step in the cosmic distance ladder.
More next month.
Lunar phases for December: First Quarter on the 5th, at 4:26 pm;
Full Moon on the 12th, at 11:37 am; Last Quarter on the 19th,
at 5:29 am; New Moon on the 27th, at 7:22 am.
Saturn is once again the only pre-dawn planet. You’ll find it high to the south,
below and to the left of Regulus in Leo, drifting slowly east relative to the constellations
as the month advances. (This may be confusing, since it also is drifting to the
west relative to the constellations! Saturn rises after midnight early in the month,
about two hours earlier by the end of December. Mars is back into the predawn skies
by the turn of the New Year, but just barely. It’s about 4 degrees above the eastern
horizon at sunrise.
At sunset, Jupiter and Venus are still bright to the southwest, close to the Moon
on the first. Venus is still moving east relative to the Sun, but Jupiter is now
drifting west (retrograde motion) and will be only about 15 degrees from the Sun
by month’s end. It will be close to Mercury, which has now moved into the early
Our midmonth view, about three hours after sunset, finds the constellation Andromeda
almost directly overhead. That faint fuzzy patch, visible on a clear and Moonless
night is the Andromeda Galaxy. Unlike most of the hundreds of billions of other
galaxies in our observable universe, it’s actually heading this way. You can, however,
relax – it won’t get “here” for another 5 billion years or so! Our own Galaxy projects
the plane of its disk across the sky from east to west, but bowed at this particular
time to the north. High above the northern horizon is the familiar shape of Cassiopeia
– now looking like a crooked M.
As winter approaches, we turn to the east to see Taurus and then Orion rising. Above
Aldebaran in Taurus lies the familiar open cluster of the Pleiades. (We mentioned
above the Hipparcos satellite. There was considerable controversy over the Hipparcos
reported distance to the Pleiades, which placed the cluster about 10% closer than
any previous measurement. That has since been resolved by carefully reanalyzing
the data. It turns out that the Pleiades were so much brighter than the comparison
field used that the distance was artificially shrunk. Using a different comparison
field corrected the result. Science is, indeed, self-correcting.) Orion’s belt is
here vertical relative to the horizon. Bright red Betelgeuse is to the left. New
data shows that this red supergiant star is plowing through the interstellar medium,
creating a huge shock wave. To the right of the belt, look for a blue supergiant,
Rigel. Turning to the west, we see Cygnus now looks the part of the “northern cross”
as it settles to the horizon.