Moon and Saturn are invitations to a worldwide Star Party
As twilight descends this evening, shining high in the southeast will be the moon and Saturn. These are the two biggest crowd-pleasers for wowing people with a telescope. By no coincidence, this is the date of the first 24-hour Global Star Party, in which more than 1,000 observatories, astronomy clubs, schools, and enthusiastic amateurs will host public sky-viewing events as nightfall sweeps around the globe.
Tonight's festivities are part of a project called 100 Hours of Astronomy, running worldwide through Sunday. It includes lectures, observatory tours, webcasts, and chances to control telescopes on far continents from your home computer. The 100 Hours campaign is part of the International Year of Astronomy, sponsored by the United Nations, marking the 400th anniversary of Galileo's first telescopic discoveries in 1609.
In Eastern Massachusetts, astronomers are putting on skywatching events in Wellesley, Westwood, Attleboro, Plymouth, and on the Cape and Islands, weather permitting. For details and late additions, zoom in on www.100hoursofastronomy.org/component/eventlist/eventsmap.
But you don't have to get in the car to tour the universe. The moon and Saturn are the nearest and farthest thing in the solar system, respectively, that you can easily see with the unaided eye. The moon is 1.3 light-seconds away. Saturn is currently 71 light-minutes from Earth, 3,300 times as distant. As a telescope will show, Saturn has moons.
The biggest and easiest to spot of these is Titan, shrouded in a smoggy, orange-brown atmosphere. In an amateur scope, Titan is a tiny dot to one side of Saturn, displaying a hint of orange. The hydrocarbon-rich atmosphere you're seeing, at a temperature of -290 degrees F, rains liquefied natural gas onto the dim, frigid lands below. The liquid forms rivers hundreds of miles long that wind through canyons down to lakes and small seas. Titan's poles are its lake country; the equatorial regions, nearly as cold, display vast expanses of dunes hundreds of feet high spaced a mile apart, made of something estimated to have the color and consistency of coffee grounds.
We're not making this up. NASA's Cassini spacecraft has been studying Saturn and its many moons from close up for nearly five years. Cassini's photos, along with reports on hundreds of findings, await your browsing at saturn.jpl.nasa.go or ciclops.org.
Our own moon is dull by comparison. But it's the only world close enough to show much detail in a a telescope. Here we get to see true alien landscapes from the back doorstep, with views of mountains, hills, plains, and craters that change aspect from hour to hour and day to day with the sun's changing illumination.
Also in the vicinity is the star Regulus, a pale blue-white point that is the leading light of Leo the Lion, outlined here. The moonlight may hide most of Leo's connect-the-dots pattern. But take a look at Leo's second-brightest star, to the upper left of Regulus. This is Gamma Leonis, one of the loveliest double stars for a small telescope. Although it looks single to the eye, it is actually two very close together. They revolve around each other every 600 or so years. Many stars are double, but this one shows its duality especially well.
Galileo's first discovery in 1609 was that the moon has rugged landscapes, contrary to all received philosophy. The next year Saturn was in view, and he did his best to understand what the misshapen thing he was seeing, but his telescopes were too poor to reveal Saturn's rings for what they are.
Later in life, he had an exciting but fruitless experience with a double star. By 1617 Galileo had been ordered by the Catholic Church not to defend or believe the "formally heretical" idea that the Earth moves. But secretly he still did. That January a friend, Benedetto Castelli, wrote Galileo an excited note that he had discovered the first known telescopic double star, in the handle of the Big Dipper. Apparently fearing a mail intercept, Castelli obliquely wrote, "I don't believe that in our pursuit one could desire better."
The pursuit was for proof that Earth circles the sun. If the two stars of the double were at very different distances from Earth, the near one would appear to move back and forth with respect to the far one once a year, as our moving observing platform circles the sun.
Despite Galileo's best efforts, nothing came of it. The stars of the double are in fact the same distance from us, so they show no parallax motion with respect to each other. All stars are too distant for their tiny parallax motions to be detectable with 17th-century instruments. The first measurements of stars' actual parallax motions had to wait another two centuries.
Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month. 
