Saturday, March 20, 2010

Spring is Young Moon Season

Wednesday's thin crescent Moon with Venus.

While the Full Moon is often considered natural light pollution, the same astronomers who hate the full version may plan, days in advance, the perfect spot to sight a Young Moon just past new. So why the change in attitude?

Young Moons are, besides quite aesthetic, rare, very rare. To sight a Young Moon under 24 hours old (and even one under 30 hours old), all the conditions need to line up just right. If everything goes perfectly, on the day after New Moon, or even on the same day sometimes, just past sunset, a wire-thin crescent will pop out low on the horizon among the Sun's last rays. Needless to say, when dealing with a Moon less than 2% illuminated, binoculars are a must.

So here is why the Young Moon is so difficult to spot:

1. Timing. If New Moon is timed too close to sunset, it will be lost in the Sun's glare on the day of New Moon and will be way past a day old come the next night. A 36 hour Moon is no challenge, pure and simple.

2. Clouds. If it's cloudy, there's no seeing the Moon.

3. Light. Young Moon hunters are forced to fight twilight. With the Moon only 1-2% lit, just the act of spotting the Moon low on the horizon in such light conditions is a challenge because that is where the Sun is. A saving grace can be (and was for me both times) a nearby planet, Mercury and Venus, respectively. If you can use a bright planet as a marker, it is a lot easier to estimate where the Moon will appear once the sky gets dark enough.

4. Haze. Even more so than during the day, haze makes its presence known at dusk, looking similar to wispy clouds on the horizon. While the biggest problem during the summer, haze can even appear in winter, too. Even a crystal-clear day can produce haze on the horizon at dusk. While the haze will quickly dissipate come dark, that's too late for the Young Moon.

These difficulties compounded with horizon issues showcase why Young Moons are the Holy Grail of Lunar observers. How rare is it to see a Young Moon? In 2006, I caught a 19 hour old Moon and it wasn't until this February that I caught another sub-24 hour old Moon, this one a meager 17 hours past new. They were always there, but one of the conditions kept me from sighting one for nearly 4 years.

Now for the good news: spring is Young Moon season. Because of the near vertical ecliptic at sunset, the waxing Moon will hang higher in the sky now than any other time of year, which is good. For Young Moon Hunters, February through May (even June depending on time of month) is an ideal time to look. By the time July rolls around, the ecliptic is undeniably flattening too much to make observing the Young Moon really feasible.

Get out while you can!

Future thin crescents:
April 15, 36 hours old
May 14, 24 hours (a Young Moon!)
June 13, 40 hours

The Spring Sky: A Cosmic Zoo

Spring is the amateur astronomer’s cure for star withdrawal after the mostly cloudy season of Winter, and the Spring skies do not disappoint. Spring can be thought of as a zoo because so many of the Spring constellations represent animals.

Of all the seasons, Spring may be the easiest with which to navigate the night sky. Smart sky watchers utilize the now well-placed Big Dipper, an asterism within Ursa Major, the Great Bear, as a signpost to other parts of the sky. Follow the line of the handle’s arc about 30 degrees from the Dipper and you’ll come to Arcturus, an orange star and the brightest star of the spring sky. Looking up, you should be able to make out, ease depending on where you live, a kite shaped pattern of stars with Arcturus as the base of the string. This kite shape is the constellation Bootes. To the East of Bootes is a small arc of stars, Corona Borealis, the Northern Crown. Going from Arcturus to Corona, continue East to arrive at the constellation of Hercules. Although large, Hercules is composed mostly of rather dim third magnitude stars, the brightest six make almost an ‘H’ shape in the sky. Off of the top four stars in the 'H' are dimmer stars representing arms and legs. Using some imagination, Hercules looks quite human. Continuing about another 30 degrees from Arcturus using the line from the Big Dippers handle, you’ll find a bright blue star, Spica, the brightest star of the constellation of Virgo, which, like Hercules, is composed of mostly third magnitude stars. From Spica, keep going about another fifteen degrees and you’ll be looking at a trapezoid of second magnitude stars that represent another animalistic constellation, Corvus the crow. An easy way to navigate the Spring sky is to "follow the arc to Arcturus, speed on to Spica, and continue the curve to Corvus."

The lion is considered to be king of the jungle. Leo, the cosmic lion, rules the Spring sky. To find Leo, look high in the south. Leo’s brightest star is Regulus, a first magnitude blue star. Going West from Leo leads you to a dim, upside down ‘Y’ shaped grouping of stars, the constellation of Cancer the crab. Although Cancer is a zodiac constellation, it is not very prominent and will be difficult to spot from the suburbs The Spring sky is home to the largest of all constellations, Hydra the snake. Just below Cancer is another test for the suburbs, an oval grouping of third and fourth magnitude stars, the head of Hydra. The body of Hydra, pardon the pun, snakes its way over 80 degrees across the sky before finally ending below Virgo.

While a time for learning new Constellations, Spring is also a great chance to get a late glimpse of Winter constellations now descending in the West. Go out just as the sky is getting dark to catch Orion, Canis Major and Minor, Gemini, Auriga, Perseus, and Taurus before they take a vacation behind the glare of the sun before eventually reappearing months later just before sunrise in the Fall.

A special bonus in the Spring is the Zodiacal Light, caused by leftover dust from the solar system formation. The solar system formed on a disc along the Ecliptic plane, on which the planets now travel. In the Spring evenings, the Ecliptic is almost vertical, allowing for the last rays of sunlight to reflect off of dust left over from planet formation. However, to see the Zodiacal Light, you must live away from the city, the deeper in the country the better. As a consolation to city dwellers, because the ecliptic is nearly vertical, Spring evenings are a prime time for seeing the planets within the Earth’s orbit, Venus and especially elusive Mercury. Spring is also the best time to spot young moons just past new, again, thanks to the nearly vertical ecliptic at sunset. Between some recognizable patterns, the Big Dipper navigation system, and the planet friendliness, spring skies have plenty to offer.

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