Don’t worry — it’s just a phase

Most diehard ama­teur astronomers hate the moon. Many stargaz­ers like to use their tele­scopes to observe celes­tial objects like star clus­ters and galax­ies. But those so-called “deep-sky” objects are faint. The bright­ness of the Moon tends to wash them out, as it washes out most of the night sky. So just as stargaz­ers often drive away from the cities to escape the glow of street lights, they also choose nights where some part of the evening isn’t spoiled by the pres­ence of the moon.

It’s fairly easy to pre­dict a good night using the phases of the moon. Here’s how to do it.

The lunar cycle is about 28 days long and begins with the new moon. Com­bin­ing a cloud­less night, a new-moon night, and a week­end night is what stargaz­ers dream of, and stargaz­ers’ spouses have night­mares about. The next New moon occurs in Sep­tem­ber, but the date varies from month to month, so it helps to have a cal­en­dar that gives the lunar phases.

At new moon, the moon doesn’t appear in the sky. As the old adage goes, “New moon means no moon. We are blessed with a full night of observ­ing from sun­set to sunrise.

In the week fol­low­ing the new moon, the illu­mi­nated part of the moon gets larger. With each night, the moon starts a lit­tle higher in the sky and sets a lit­tle later. The moon­less observ­ing period gets any­where from a half hour to a full hour shorter, depend­ing on the sea­son. Also, the chance to observe occurs later and later in the evening.

In Sep­tem­ber, by the time the moon reaches first quar­ter (the “half moon”), a week after new, it doesn’t set until about 11:30 p.m. That makes for a very late night. You won’t get in from observ­ing until sun­rise, which can cause domes­tic or employ­ment difficulties.

Many is the time I’ve walked into an 8 a.m. class to teach look­ing like a zom­bie newly risen from the ground, and my stu­dents have whis­pered know­ingly to each other, “He’s been observ­ing again — stargaz­ing from beyond the grave.” (Note to my boss: I don’t do that any more. Honest.)

Dur­ing the week after first quar­ter, the moon keeps get­ting big­ger, and the moon­less period keeps shrink­ing. In Sep­tem­ber, by four days after first quar­ter, the moon doesn’t set until 3:30 a.m., and morn­ing twi­light begins around 6 a.m.

The moon grows until it reaches sec­ond quar­ter, or full moon, a week after first quar­ter. At this point observ­ing any­thing except the moon and bright plan­ets is impos­si­ble because the moon rises around sun­set and sets around sunrise.

In the week after it is full, the moon pro­gres­sively shrinks until it reaches third quar­ter (another half moon). Dur­ing that period, the moon has not yet risen at sun­set and rises a bit later each night. In Sep­tem­ber, it takes the whole week for the later set­ting time to work its way past evening twilight.

How­ever, by third quar­ter you’ll get a good two and a half hours of dark-sky observ­ing time.

After third quar­ter, the moon is mov­ing toward new again, con­tin­u­ing to shrink until it is a thin sliver in the sky. It rises later and later in the night, mak­ing for longer and longer nights of observ­ing. A week after third quar­ter, the moon is new again and dis­ap­pears. The cycle begins anew.

My favorite time of the lunar month to go stargaz­ing is the period between third quar­ter and new moon. I can observe in the early evening and get home early so my wife doesn’t have time to change the locks. It also means a decent night’s sleep, so my boss doesn’t ask, “Isn’t that your twelfth cup of cof­fee?” as I slink back to my office from the java machine.

Plan­ets

Sep­tem­ber is a great month to get up just before morn­ing twi­light. The two bright­est plan­ets, Venus and Jupiter, are low in the south­east, Jupiter above and Venus below. To the right, look for the bright star Betel­geuse. The three form a very nice triangle.

Dur­ing the morn­ing of Sept. 8, the cres­cent moon is very close to Jupiter. The moon then slips down between Jupiter and Venus. Luna gets close to Venus on Sept. 12. On that morn­ing, look for the Bee­hive Clus­ter as a hazy patch on the oppo­site side of Venus from the moon.

Tom Burns is the direc­tor of Ohio Wes­leyan University’s Perkins Obser­va­tory. He can be reached at tlburns@owu.edu.

Posted by Tom Burns on Aug 26 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.