March 28, 2010

Sky Chart: What's Up in April

Yes, it's a month with plenty to watch in the heavens

By BERNIE REIM

The name for April comes from the word Aprilis, which means aperture or opening. That's what much of the northern hemisphere is doing as we are ever so slowly tilting back toward the sun again after the vernal equinox.

click image to enlarge

This chart represents the sky as it appears over Maine during April. The stars are shown as they appear at 10:30 p.m. early in the month, at 9:30 p.m. at midmonth and at 8:30 p.m. at month’s end. Mars and Saturn are shown in their midmonth positions. To use the map, hold it vertically and turn it so that the direction you are facing is at the bottom.

Sky chart prepared by George Ayers

There are many highlights to enjoy during the warmer nights. Try to find the darkest skies to catch these celestial events, both the obvious and more subtle ones.

We will start with the best apparition of Mercury for the whole year. Our smallest planet will be more than 10 degrees above the western horizon half an hour after sunset during the first two weeks of April. Through a telescope you will notice that Mercury is slowly getting less illuminated by the sun, but larger in the sky as the month progresses. It will appear like a last quarter moon on the 10th, then turn into a waning crescent.

Legend has it that Copernicus, who formed the heliocentric theory nearly 500 years ago, never actually saw Mercury. He most likely did see it during its better apparitions, but Mercury is the lowest and faintest of the 5 naked-eye visible planets, and many people on Earth still never have seen it.

Venus, will drift just 3 degrees above Mercury during the evening before Easter and also the next evening, Easter Sunday, April 4. Venus is continuing to slowly climb higher into the sky even as it gets a little larger and less illuminated by the sun as it gets closer to Earth on its faster orbit.

We will welcome back Jupiter into the morning sky an hour before sunrise. the end of April, it will rise two hours before the sun.

The slender waxing crescent moon will join Mercury and Venus one hour after sunset on the 15th, then proceed to drift past the Pleiades again during the next two evenings.

The last obvious highlight will be Mars gliding just above the Beehive star cluster again during the evenings of the 16th through the 18th. Try to watch this event through binoculars or a telescope. See if you can observe the continual, second-by-second motion of Mars near the stars in the Beehive through a telescope. That is possible because Mars moves quite fast, half a degree eastward per day. That is only 24 times slower than the moon, which moves its own width, half a degree, eastward every hour. The red planet ended its retrograde, or westward motion toward Gemini on the 11th of last month, and is back to its normal, prograde, or eastward motion toward Leo. Watch as the first-quarter moon drifts just below the celestial pair shortly after dark on the evening of the 21st.

The first subtle event of April will be your last chance to see the zodiacal light in the evening sky this year. It only shows up about one hour after sunset around the beginning of spring or one hour before dawn in the morning sky in fall. I have only seen it twice, once before dawn during that spectacular and memorable Leonid meteor shower on Nov. 18, 2001, when it was literally raining meteors for three hours straight at the rate of about 1,000 per hour. The other time I saw this amazing event was about one hour after sunset in March during a star party that our astronomy club gave in Cumberland.

The zodiacal light looks like a tilted cone of ghostly, glowing light in the shape of a haystack or pyramid. Right now it would be centered on Venus and may reach up as high as the Pleiades and Hyades star clusters. Start looking for it just as the last of the evening twilight fades away from a very dark-sky site with no light pollution to mask this subtle beauty. The zodiacal light is an interplanetary dust cloud made up of comet dust and tiny leftover particles from the formation of our solar system 4.6 billion years ago. These particles are several miles apart, but they pervade the plane of our solar system and reflect light well. Remember it's always there, but we can only see it for a few weeks twice a year as the angle of our horizon to the ecliptic is at its steepest.

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