September 2011 Sky from the Keeble Observatory
Last month we promised to say something about two separate events. The first was
the arrival at the main belt asteroid known as 4 Vesta of the Dawn space probe.
The other was the discovery of a fourth moon orbiting distant (and demoted from
planet status) Pluto. The latter should not surprise us, since we know of other
Kuiper Belt objects with their own moons. (The Kuiper Belt is a toroidal
reservoir of comets and dwarf planets – some are even larger than Pluto, which is
part of why it was demoted to dwarf planet status. This region lies beyond the orbit
of Neptune, and includes orbits between 30 and 1000 times the size of Earth’s orbit.)
Here’s a picture of this little system of Pluto and its four moons, but one of the
things we learn is that Pluto is hard to observe even with the Hubble Telescope!
Image courtesy NASA, ESA, and M. Showalter (SETI Institute)
The New Horizons probe, launched in 2006, will be in the Pluto system in 2016, and
we hope to get close up images.
Dawn is the first probe to orbit a main belt asteroid. 4 Vesta was the fourth discovered,
and as more and more were found, it led to the proposal that asteroids were remnants
of an exploded planet. It went into orbit on July 16, and has started returning
images and taking measurements of the gravitational field and composition of the
asteroid. Here’s one of the first images returned from orbit at an altitude of about
Image courtesy NASA/JPL-Caltech/UCLA/MPS/DLR/IPA
We can see here that with a full year to study Vesta, there will be lots of interesting
questions. For example, just what’s going on in that weird grooved terrain around
the equator (upper left)? After that year, during which Dawn’s orbit will slowly
spiral closer to the asteroid, controllers will restart the ion engine and send
Dawn on its way to another main-belt target: 1 Ceres, which is large enough to qualify
as a dwarf planet rather than an asteroid.
Lunar phases for September: First Quarter on the 4th, at 1:39
pm; Full Moon on the 12th at 5:27 am; Last Quarter on the 20th,
at 9:39 am; New Moon on the 27th, at 7:09 am.
I’ve had several questions about why I give times as well as dates for lunar phases.
This is because the phases technically mark a specific angular relation between
Sun and Moon, defined in terms of their ecliptic longitude – which is the
distance between them measured along the Sun’s path, i.e. the ecliptic. Remember
that they are both moving to the east against the background sky, but at different
rates. New Moon occurs at the precise moment when the center of the Moon’s disk
and the center of the Sun’s disk have the same ecliptic longitude. The Moon may
be above or below the ecliptic when this happens. If it’s on the ecliptic, we get
a total solar eclipse. Similarly, when they are 180 degrees apart in ecliptic longitude,
we have a Full Moon. If it’s on the ecliptic, we get a lunar eclipse. First quarter
is when the Moon is 90 degrees east of the Sun along the ecliptic, Last Quarter
when it is 90 degrees west.
Predawn planet watchers will have to look carefully for Mercury, low in the east
early in the month. It will disappear into the Sun’s glare by midmonth. Mars is
higher (about 40 degrees) and bright red. Don’t confuse it with Antares, also bright
red but a star instead of a planet. Antares is about 30 degrees to the right of
Mars. Turning to the south, you’ll see Jupiter high above the horizon. By month’s
end, Mars will be found about five degrees further west, and Jupiter will have moved
to the western skies.
Early evening planet watchers will find Saturn low to the west as it emerges from
evening twilight. Say good bye, since our orbit is carrying us around the Sun faster,
and the Sun will soon be along that line of sight. Jupiter will rise to the east
about 3 hours after sunset early in September, and shift slowly to about 2 hours
after sunset by month’s end.
Our mid-month view of the sky, about two hours after sunset, finds Cygnus (the Swan)
at zenith, appearing to glide along the Milky Way from northeast to southwest. The
bright star Deneb marks the tail of the swan, about ten degrees northeast of zenith.
At a similar angle to the west we see brilliant Vega in the constellation Lyra.
These two bright stars make a triangle with Altair, about 30 degrees south of zenith.
With binoculars, on a clear dark night, just east of Deneb you may find the ghostly,
familiar outline of the “North America Nebula. Cassiopeia is to the northeast, looking
like a W tipped backwards. If you follow the line of the “left” side of the W toward
the east you’ll notice the faint fuzzy patch of the Andromeda Galaxy, about 45 degrees
above the eastern horizon. It’s best seen on a clear, moonless night, and best by
“averted vision” without binoculars. That you can see it at all gives some sense
of the scale of our Universe, since this is the most distant object visible to the
naked eye. It’s practically in our back yard! This spiral galaxy is only 2.2 million
light years away, and is on a collision course toward our home Galaxy … it will
be “here” in about 5 billion years, roughly when the Sun becomes a red giant!
If readers have questions about astronomy or science in general that you would like
covered in one of these columns, please feel free to contact me at email@example.com.