Physics Department

Keeble Observatory

June 2003 Sky from the Keeble Observatory

Mars, which rises at midnight this month and, perhaps, the most intriguing of our celestial neighbors, is about to receive some visitors! Actually, the visitors aren't arriving until early next year, but they are about to leave on the journey from Earth. In fact, about every two years you may have noticed a flotilla of probes launching to the red planet . some, like Pathfinder and the Sojourner rover, spectacularly successful. Others have been embarrassing failures.

Next month we'll discuss the array of missions in store.

This month, I want to address the question, "Why every two years?" A sidereal year on Earth, the time for our planet to return to the same position in its orbit, relative to both Sun and stars is 365¼ days. For Mars, we measure two different "years" . the sidereal year (relative to the stars) is just a few minutes under 687 Earth days. But, because both Earth and Mars are moving in their orbits, we can also measure a synodic year, the time for Mars to return to the same position relative to Sun and Earth, is 779.9 days . a little less than 26 months.

Imagine the Earth's orbit as a circle of radius 6". On that scale, Mars' orbit would be drawn as a concentric circle of radius a little more than 9". Now, imagine drawing an ellipse that "kisses" both circles at the ends of its long axis. That ellipse - called a transfer orbit - marks the most cost-efficient (in terms of rocket power) trajectory to get from Earth to Mars. The trick is to launch at such a time when Mars will be at the outer end of the ellipse when the probe arrives! Those relative positions occur when? Every 26 months - the synodic period! We can go a little more energetic, or a little less, so there is a launch window of several months when it is economical to send probes to Mars . hence the roughly two-year intervals between missions. Time to arrive is intermediate between half of Earth's year, and half of Mars' year . so arrival is in 7 - 9 months after launch.

Lunar phases for June: First Quarter on the 7th, 4:29 pm; Full on the 14th, at 7:17 am; Last Quarter at 10:46 am on the 21st; New Moon at 2:40 pm on the 29th.

Mercury and Venus remain low in the pre-dawn sky . look for them to the east before sunrise.

We'll lose Saturn to the Sun's glare by month's end, but Jupiter remains bright in the western evening sky all month. Early in the month, look for it about 45 degrees off the horizon just after evening twilight, and follow it as it settles closer to the horizon throughout June.

About an hour after sunset at mid-month will give you what seems a fairly open sky. The brightest star near zenith is Arcturus, but most of the bright stars are closer to the horizon. On a clear night, you might look with binoculars, sweeping slowly from Arcturus to zenith . on the way you may glimpse the bright globular cluster known as M3, a few million stars in an old cluster orbiting the Milky Way. On a really clear, moonless night, you may even discern this cluster without binoculars! If you start with binoculars on the end star in the handle of the inverted "Big Dipper" look just a bit to the west for the spiral galaxy M51 . you're not likely to catch this without binoculars or telescope. To the northeast you'll notice a bright triangle of stars: Vega is the highest, with Deneb lower and further to the right, Altair lower still and to the left. These three bright stars mark the "summer triangle" which will be high overhead later at night, and later through the summer months. That bright red star to the southeast is Antares . don't confuse it with Mars, which you remember doesn't rise until midnight. Higher to the south you'll see bright Spica, in Virgo. To the west, above and to the left of Jupiter is Regulus, in the heart of the constellation Leo.

For your own monthly star chart, you can direct your web browser to http://www.skymaps.com. You will find extensive descriptions of what's worth looking for, and you can download and print a single copy for your personal use.

Copyright 2003
George Spagna