Wednesday, April 1, 2015

The April Sky

With the new month of April here, the lengthening of the days will continue at a brisk pace as the equinox is still within recent memory. By month's end, staying up late to observe under dark skies will become a chore for many. On the good news front, for much of the country, April is usually the first month of the year where the persistent winter clouds finally break with regularity, revealing the stunning celestial sights that had been so often shrouded in clouds for the first 3 months of the year.

Cool Constellations
If March was the last time to see many winter constellations under dark sky conditions, April is the last chance to see them, period. So, come sundown, look low in the West-Southwest for Cassiopeia, Cepheus, Perseus, Orion, Canis Major, and Canis Minor. A few later/more Northerly winter constellations, including Auriga, Taurus, Gemini, and the Pleiades are now in that twilight zone where one needs to observe them under dark skies now or miss them for the year. By the time April rolls around, the Big Dipper signpost to the stars is well up for all to see. Starting at the Dipper, follow the arc of the Dipper to bright orange Arcturus, alpha Bootes, and the brightest star in the spring sky. To the left of the kite-shaped Bootes, look for the arc of stars that is Corona, the crown. Next, speed onto blue Spica, alpha Virgo, and one of the brightest spring stars. Next, continue the curve to trapezoidal constellation Corvus, the crow. Finally, conclude in dim Crater the cup. Moving higher in the sky, zodiac constellations Cancer and Leo are well-placed as well. Speaking of Cancer, look just below the cosmic crab for a distinct ring of stars, the head of Hydra, the sky's biggest constellation, which snakes (sorry) through over 120 degrees of sky. For those who like to stay up late (or get up extremely early), there's mythological strongman Hercules and the Summer Triangle high overhead and, to the South, Ophiuchus, Serpens, and Scorpius. However, unlike in previous months, the later time of year will not bring new predawn constellations thanks to the Sun, which is rising dramatically earlier by month's end.

Planetary Perceptions
Like March, April is looking to be another 5 for 5 planet month. At dusk, Earth's two planetary neighbors, Mars and Venus, will continue to be easy dusk objects. As was the case last month, Venus will continue to climb out of the dusk twilight while Mars will begin to slowly sink in the Southwest, setting roughly 2 hours after the Sun at month's start but only about an hour after come month's end. Moving later into the night, Jupiter is well-placed for prime-time viewing as it rides high and due South at true nightfall. Additionally, Saturn continues to re-emerge from the Sun's glare in the predawn sky. By month's end, the ringed wonder will be just about due South at dawn. Last but not least, speedy Mercury will reemerge from the Sun's glare starting around mid month at twilight in what is shaping up to be its best evening appearance of the year, which will take place in the April to May transition.

Young Moon Season
Want to find the Holy Grail of naked-eye astronomy? Well, you're in luck as spring is Young Moon season, which runs through June and features wire-thin, barely there crescents just after sunset. The funny thing: many people who call Full Moons light pollution will plan young Moon hunts weeks in advance. 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. Now for the good news: spring is Young Moon season. Because of the near vertical ecliptic at sunset, the Young Moon will hang higher in the sky now than any other time of year, which is good. For Young Moon Hunters, March 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 19: 31 hour Moon
May 18: 22 hour Moon
June 17: 36 hour Moon

Interesting Astronomical Tidbit: Thanks to convenient timing, the Moon will reach a point in its orbit called apogee twice this month. For more details, look below . . .

Tonight's Sky for April 1: Moon at Apogee
Tonight, the Moon is about as small as it will ever get thanks to the fact that the Moon is at apogee, a point in its orbit that is farthest from Earth.

What many people may not realize is the fact that the Moon (and all other celestial bodies) do not orbit their parent bodies in circles, but ellipses, which are slightly elongated circles. Result: any given day of an orbital period, any orbiting body will be at a slightly different distance from its parent body. As for the Moon, this variance in orbit amounts to about 20,000 miles.

As for tonight, the Moon will be about as far from Earth as it is going to get. When it comes to practical implications, the difference will be hard to notice with the naked eye to all but an experienced observer but, in a telescope,
the difference will be obvious.

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