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.
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 email@example.com.