The star of the month for May is the spectacular planet Saturn. Well placed for long views in any telescope on warm nights, Saturn never leaves anyone disappointed. Then, at month's end, three other major planets join to form a striking grouping in evening twilight.
Its rings are Saturn's most noticeable feature. During its 29 year solar orbital period, Saturn's rings are viewed tilted toward our line of sight at varying angles. The planet is currently halfway between the two extremes. The rings appeared edge-on in 2009 and will be viewed at their widest in 2017.
The first recorded observation of Saturn's rings was in 1610 by Galileo Galilei. However, Galileo's small and, by today's standards, crude telescope did not reveal the nature rings. He interpreted what he saw as a triple planet - a large disc with a smaller one on either side. He also described the latter as "ears."
Two years later the rings were edge on and no longer visible in Galileo's instrument. They then returned a year later. Galileo found these events to be inexplicable.
In the 1650s, Christiaan Huygens used an improved telescope to resolve the nature of the rings. He observed them as being thin or "flat" and correctly deduced that Saturn's axis is tilted and that our viewing it at different positions as it revolved around the sun accounted for the varying tilt in the rings' appearance.
Even at low power, small telescopes will reveal the ringed nature of Saturn. For centuries there were thought to be only three rings. The A and B rings are the brightest, with the former being farthest out from the planet. The faint C or "crepe" ring is the next one in, closer to the planet than B.
In 1676, astronomer Giovanni Cassini discovered a dark gap between the A and B rings. This gap, known as the Cassini Division, is visible in moderate sized telescopes. It's best observed at the far edges of the rings on either side of the planet.
Saturn has dozens of moons. The largest moon is Titan. It is larger than our moon and its diameter would reach from the tip of Florida to Washington state's pacific coast. Cold Titan has a dense atmosphere and on its surface are lakes of liquid methane. The Huygen's probe measured Titan's surface temperature at -290øF.
Like the rings, Titan is another of Saturn's features visible in small telescopes. Look for a faint star that stays close to the planet and is usually in the same eyepiece field.
The larger the telescope, the more moons will be visible. A javascript utility showing the appearance of Saturn, orientation of its rings, and locations of five of its brightest moons is available from SkyAndTelescope.com. Just enter in the expected time of observation and press update.
A chance to observe an interesting grouping of three major planets during evening twilight occurs on May 26 and 27. The planets are Mercury, Venus, and Jupiter. Plan to begin looking around 20-30 minutes after sunset, say about 8:45-9:00 p.m. EDT, and be patient.
Jupiter will be to the left of Venus -- the brightest of the trio. Fainter Mercury is above Venus. The configuration will slowly change over the course of several days. The trio will appear very low to the horizon, so make sure you have a clear view. Use binoculars to help pick out the planets in the twilight.
