Keeble Observatory
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 3200 miles:
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 gspagna@rmc.edu.
Copyright 2011
George Spagna