Friday, May 1, 2015

The May Sky

With the arrival of May, the news is of a very mixed nature for astronomers in much of the USA. First, the bad. While the lengthening of the days will slow but that is little consolation for astronomers as, by this time of year, staying up for a dark sky often comes at the cost of being groggy for work in the morning. However, the good news is threefold. First, the rate of cloudiness will drop as it did last month. Second, May will finally bring consistently warm nights where bundling up is no longer a must. Third, at least at the start of the month, the bugs shouldn't be much of an issue, either.

Cool Constellations
By the time May arrives, most of the winter constellations are a memory. Still hanging around low in the West are Auriga and Gemini, but they won't last much longer. Also, barely circumpolar Cassiopeia is now just scraping the Northeastern horizon, too. By the time May comes around, there is no better time than now to use the handy Big Dipper signpost. 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 and looking in the South-Southwest, zodiac constellations Cancer and Leo are still well-placed, but this will be ending come next month. 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 later, there's mythological strongman Hercules and the Summer Triangle high overhead and, to the South, Ophiuchus, Serpens, Libra, and Scorpius are all starting to rise at a somewhat reasonable hour. By the time the sky starts to brighten, Vega is at Zenith, Sagittarius is due South, and the Great Square has reemerged in the East.

Planetary Perceptions
Like April, May is looking to be another 5 for 5 planet month, albeit with a couple of catches. The real treat for May, 2015 will be Mercury, which will be putting on its best evening show of the year this month, peaking in elevation around the 8th. Unfortunately, as is the norm with good appearances, once the little planet starts to drop, it will in short order disappear from the sky by 2 weeks after greatest elongation. Additionally, Mars, which has been hanging around low in the Southwest for the past several months, will finally drop out of visibility in May. On the other hand, Venus will continue to climb out of the dusk twilight all month long. 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 in the Southwest at dawn.

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:
May 18: 22 hour Moon
June 17: 36 hour Moon

The Big Dipper: Signpost to the Stars
Late spring and early summer presents the Big Dipper at its best just as the sky is getting dark. While many people not familiar with astronomy have heard of the Dipper, many people familiar with astronomy do not realize that it makes for a handy guide to the late spring/early summer sky. Starting at the Dipper, follow the arc of its handle to bright orange Arcturus, alpha Bootes and brightest star of the spring sky. From Arcturus and following the same line, speed on to Spica, Virgo's not quite as bright blue alpha star. From there, continue to Corvus, a small, though rather conspicuous trapezoid-like constellation low in the Southern sky and, from there, conclude in Crater, the dimmer, neighboring constellation to Corvus. Moving back to the Dipper, follow the imaginary line created by the stars representing the end of the bowl to Polaris, the North Star. Continuing that line about the same distance through the celestial pole will bring you to 'W'-shaped Cassiopeia, which is just scraping above the Northern horizon this time of year for us at mid-Northern latitudes. Besides aesthetic, the Dipper's pretty practical, isn't it?

Tonight's Sky for May 1: May (and Cross-Quarter) Day
Today is May Day and also a cross-quarter day. What is a cross-quarter day? It's the mid way point of any season and was, like the solstices and equinoxes, an important time for early civilizations as these days served as another natural way to divide up the year into periods of time, which became a matter of life and death with the advent of large scale agriculture around 5000BC.

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