Observer's Notebook for May, 2000

Spotting the Young moon

by John Rummel

Sometimes simple pleasures in astronomy are the best. This month's challenge is an example.

Sky-watchers speak of the phases of the moon in terms of new, full, first- quarter, waxing crescent, etc. Though the terminology may seem confusing, the concepts involved are relatively easy to grasp.

As the moon revolves about the Earth each month, its phase changes nightly as its orbit carries it further from the sun in our sky. New moon occurs when the moon and the sun are so close in the sky, that the moon's illuminated face is turned completely away from the Earth, thus, we see no moon.

Full moon occurs approximately 14 days later when the moon has moved through about one half of its orbit, so it is now opposite the sun in the sky, and thus its illuminated face is turned directly toward the earth. Since it is opposite the sun, the full moon rises as the sun sets, and is up all night long.

Fourteen days after the full moon, we're back at new moon and the cycle repeats itself. The 14 days from new moon to full moon are called waxing phases (the moon appears to get bigger each night), and the 14 days from full moon to new are called waning phases (getting smaller).

This month's challenge involves the attempt to see the youngest moon possible - that is - the moon as soon as possible after the new phase, when it's still a very thin crescent. This can be accomplished by first knowing exactly when the new moon occurs, down to the hour, and then picking the first sunset within 24-36 hours after that moment. Any moon younger than about 24 hours is very difficult to spot - it's still very close to the sun, and it's an extremely thin crescent. Moons between about 30 and 40 hours old are much easier, and the subject of this month's challenge.

The June 2000 new moon occurs at exactly 7:30 am on June 2nd, Madison time. Therefore, the moon at sunset that evening will be too young, only about 13 hours. Our challenge is the following day, June 3rd, starting at sunset.

For Madison viewers, the sun will set on June 3rd at about 8:30 pm. Find yourself a comfortable spot to sit outside where you have a nice view of the western horizon and the sunset. Use a landmark to identify the spot where the sun disappears below the horizon. Start from that spot and draw an imaginary line upward from the sunset at a 45 degree angle to the left. Now hold out your left hand at arm's length and extend your fingers as wide as they'll go. Put the tip of your thumb at the sunset spot, and the tip of your pinky at the opposite end of that 45 deg line slanting up and to the left. The area around your pinky is where you'll start hunting for the razor thin crescent moon.

Though you can certainly use only your eyes for the search, I highly recommend binoculars. You'll be able to spot the thin crescent much easier with the added magnification and light gathering power.

To assist you in your search, I've arranged to have a bright star appear very close to the moon's position on this night. As the sun sinks lower below the horizon and dusk deepens, a single bright star will appear low in the west in the general vicinity of your search. The moon will be about two finger widths (with hand held at arm's length) below and slightly to the left of the star. With a decent pair of 7x40 or 7x50 binoculars, put the star in the upper right of the binocular field, and you should be able to see the moon, with barely 3% of its surface illuminated, in the lower left.

If you spot it, congratulations, you're seeing a moon under 40 hours old, maybe the youngest you've ever seen. Now in the months to come, check the time of new moon and try to beat your record.

What's the world record for the youngest moon spotted visually? There's no official Guinness book standard, but using some fancy equipment and calculations, some pretty extraordinary results can be achieved.

Sighting the young moon is not just an amusing pastime for amateur astronomers. The Islamic calendar is based on lunar months, which begin when the thin crescent moon is visually sighted. Islamic groups around the world take young moon sightings very seriously.

By the way, that star you used to help find the moon on June 3rd? It's not a star at all, but the planet Mercury, making one of its best appearances of the year!

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