This Week's Sky at a Glance, April 4 – 12

 

FRIDAY, APRIL 4

■ First-quarter Moon (exactly so at 10:15 p.m. EDT). The Moon shines under Castor, Pollux, and Mars, forming a triangle with the line of three as shown in the view below. The scene is for North America in late twilight when the stars come out. As the night progresses and the sky turns westward, this arrangement will rotate clockwise as you turn to keep facing it — quickly at first, then more slowly. (Extra credit: Why? Answer is in footnote ¹ at the bottom of this page.)

SATURDAY, APRIL 5

■ Now the Moon, barely past first quarter, forms a new pattern with the Mars-Pollux-Castor trio as shown above. The Moon is 3° or 4° from Mars during evening in the Americas.

SUNDAY, APRIL 6

■ The Big Dipper stands high in the northeast these evenings, tipping over to the left dumping water. Its handle plus the two stars of its bowl closest to the handle form a broad, rough arc. To the right of the arc, not too far from its focus, shines a lone 3rd-magnitude star. That's Cor Caroli, the brightest star of Canes Venatici. It's a beautiful telescopic double, magnitudes 2.8 and 5.6, separation 19 arcseconds. They're spectral types A0 and F2, respectively.

Also in CVn is one of the brightest deep-red carbon stars in the sky. Y Canum Venaticorum, also known as La Superba, varies between magnitudes 4.6 and 5.9. Currently it's visual mag 5.3. Matt Wedel calls it "arguably the finest carbon star for binocular users" in his Binocular Highlight column in the April Sky & Telescope, page 43. Carbon stars like this are so red because we see them through red filters: their atmospheres are rich in diatomic carbon, C₂ , a red gas.

MONDAY, APRIL 7

■ The Moon is in Leo with Regulus 5° or 6° below it this evening. Gamma Leonis, less bright, is a bit farther left or lower left of the Moon. Those are the two brightest stars of Leo's Sickle. The entire Sickle extends upper left from Regulus by a little more than a fist at arm's length.

To see its stars better through the moonlight, cover the Moon with your hand. Or use two fingertips, one for each eye. Close one eye and position a fingertip on the Moon. Then switch eyes and position another fingertip on the Moon. Open both eyes.

TUESDAY, APRIL 8

■ Now the Moon shines below Regulus, Gamma Leonis, and Leo's Sickle.

■ The huge, bright Winter Hexagon is still in view early after dark, filling the sky to the southwest and west. It's the biggest well-known asterism in the sky.

Start with brilliant Sirius in the southwest. It's the Hexagon's lower left corner. High above Sirius is Procyon. From there look higher upper right for Pollux and Castor (lined up nearly horizontal, with Mars to their left). Go lower right from Castor to Menkalinan and then bright Capella, lower left from there past Jupiter to Aldebaran, lower left to Rigel at the bottom of Orion, and back to Sirius.

The Hexagon is somewhat distended. But if you draw a line through its middle from Capella to Sirius, the "Hexagon" is fairly symmetric with respect to that long axis.

WEDNESDAY, APRIL 9

■ In late twilight or thereabouts, Arcturus, the bright Spring Star climbing in the east, stands just as high as Sirius, the brighter Winter Star descending in the southwest (for skywatchers at mid-northern latitudes). Those are the two brightest stars in the sky at this time.

But Capella is a very close runner-up to Arcturus! Spot it high in the northwest.

THURSDAY, APRIL 10

■ Right after dark, Orion is still fairly well up in the southwest in his spring orientation: striding down to the right, with his belt horizontal. The belt points left toward Sirius and right toward Aldebaran and, farther on, the Pleiades. Jupiter grabs your eye above Aldebaran.

■ Mars, Pollux, and Castor very high in the west form a straight line tonight. Watch it begin to bend in the coming days.

FRIDAY, APRIL 11

■ At this time of year, the two Dog Stars stand vertically aligned around the end of twilight. Look southwest. Brilliant Sirius in Canis Major is below, and Procyon in Canis Minor is about two fists above.

SATURDAY, APRIL 12

■ Full Moon (exact at 8:22 p.m. EDT). The full Moon of April always shines in the vicinity of Spica. This year they're especially close together, as indicated above.

The Moon will occult Spica for most of South and Central America and the southernmost tip of South Africa. Map and timetables.

SUNDAY, APRIL 13

■ Vega, the bright "Summer Star," rises in the northeast late these evenings. How early or late depends on your latitude and also on your longitude within your time zone.

Exactly where should you watch for Vega to come up? Spot the Big Dipper almost overhead in the northeast. Look at Mizar at the bend of its handle. If you can see Mizar's tiny, close companion Alcor (binoculars show it easily), follow a line from Mizar through Alcor all the way down to the horizon. That's where Vega will make its appearance!

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This Week's Planet Roundup

Mercury and Saturn are all but undetectable very deep in the glow of sunrise; both are about 6° or 7° below Venus all week. Mercury brightens to a mere magnitude +1.0 near week's end, while Saturn holds at magnitude +1.2. Maybe try for them with a telescope if you have a very flat east horizon?

Venus is some 200 times brighter than those two at about magnitude –4.5. It's emerging low in the dawn, having passed inferior conjunction on March 22nd. Look for Venus very low due east starting maybe 50 minutes before sunrise.

Venus gets a little higher and easier morning by morning. It's in its dramatic crescent phase in a telescope or steadily braced binoculars. Post-conjunction, the bulge of the crescent now faces lower left toward the Sun. This may look odd if you're used to Venus's opposite look when it's an evening crescent after sunset.

Mars (about magnitude +0.5) is roughly in line with heads of Gemini this week. It comes into view in evening twilight as a steady yellow-orange spark high toward the southwest. It continues to fade as it shrinks into the distance, looking ever more like its neighbors Pollux and Castor: magnitudes +1.1 and +1.6, respectively.

These three dots form an arc that gradually straightens out day by day, to form a straight line on April 10th. After that the arc will start to bend the other way, as Mars progresses east against the starry background.

For telescope users Mars has shrunk to only 7½ arcseconds in diameter. Good luck making out any surface markings aside from (maybe) the North Polar Cap. At least you can see that its disk is definitely gibbous: 90% sunlit. This is as gibbous as it's going to get this season.

Jupiter shines bright white (magnitude –2.1) in the west these evenings, in Taurus about 39° lower right of Mars along the ecliptic. This week Jupiter forms an equilateral triangle with Taurus's two horntip stars above it: Beta and lesser Zeta Tauri.

Below Jupiter shines orange Aldebaran. It and the triangle of Mars and the Taurus horns together form a lower-case y. Farther to Jupiter's lower right are the Pleiades.

Jupiter sets in the west-northwest around midnight daylight-saving time.

In a telescope Jupiter has shrunk to only 35 arcseconds wide, nearly as small as it gets, as Earth pulls far ahead of it in our faster orbit around the Sun. For the daily doings of its Galilean moons see the April Sky & Telescope, page 51.

Uranus, magnitude 5.8 on the Taurus-Aries border, sinks away in the west-northwest after dark, 6° or 7° below the Pleiades.

Neptune is hidden in the glare of dawn behind Mercury and Saturn.

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All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions and graphics that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time minus 4 hours. UT is also known as UTC, GMT, or Z time.

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Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For a more detailed constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.

For the attitude every new amateur astronomer needs, read Jennifer Willis's Modest Expectations Give Rise to Delight.

Once you get a telescope, to put it to good use you'll need a much more detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas, in either the original or Jumbo Edition. Both show all 30,000 stars to magnitude 7.6, and 1,500 deep-sky targets — star clusters, nebulae, and galaxies — to search out among them.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many, as well as many more deep-sky objects. It's currently out of print, but maybe you can find one used.

The next up, once you know your way around well, are the even larger Interstellarum atlas (with 201,000+ stars to magnitude 9.5, and 14,000 deep-sky objects selected to be detectable by eye in large amateur telescopes), and Uranometria 2000.0 (332,000 stars to mag 9.75, and 10,300 deep-sky objects). And read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.

You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner. The pinnacle for total astro-geeks is the new Annals of the Deep Sky series, currently at 11 volumes as it works its way forward through the constellations alphabetically. So far it's up to H.

Can computerized telescopes replace charts? Not for beginners I don't think, and not for scopes on mounts and tripods that are less than top-quality mechanically. Unless, that is, you prefer spending your time getting finicky technology to work rather than learning how to explore the sky. As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand and a curious mind." Without these, "the sky never becomes a friendly place."

If you do get a computerized scope, make sure that its drives can be disengaged so you can swing it around and point it readily by hand when you want to, rather than only slowly by the electric motors (which eat batteries).

However, finding faint telescopic objects the old-fashioned way with charts isn't simple either. Do learn the essential tricks at How to Use a Star Chart with a Telescope.

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Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the naked-eye heavens above. It's free.

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"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
            — Carl Sagan, 1996

"Facts are stubborn things; and whatever may be our wishes, our inclinations, or the dictates of our passion, they cannot alter the state of facts and evidence."
            — John Adams, 1770

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1. Why do constellations appear to twist fast near the zenith and slower when they're lower? Because when a celestial object passes near the zenith, you have to turn faster to keep facing it.

Or to put it another way, the direction down from a sky object — that is, the direction away from the zenith point — changes a lot when the object moves sideways even a little near the zenith.

There are subtleties here. In ordinary life we think of sky objects in alt-azimuth coordinates: up-down and side-to-side. These are set by our up-down gravity and our side-to-side horizon.

The sky, however, rotates according to the celestial coordinate system, which is tilted with respect to our alt-azimuth worldview. This means that if a celestial object passes near the zenith — the pole of the alt-azimuth system — it changes azimuth (compass direction) quickly. So you have to turn yourself faster to keep facing it.

This would not happen if you turned yourself from celestial east to west instead, as you would if you lashed yourself lengthwise to a diagonal rotating stake aimed at the north celestial pole like the polar axis of an equatorial mount. If you did that and watched a constellation move for hours, slowly turning the stake to keep the constellation in front of you, it would not appear to rotate with respect to you at all.²

P.S. That zenith-twist effect is stronger the closer you live to Earth's equator. At the equator itself, a constellation that crosses the zenith rises straight up due east, goes straight down due west, and flips 180° when it crosses the zenith and you turn around 180°.

Seen from Earth's poles, the constellations just circle around you without changing altitude at all.

2. I once met someone who actually did something like that and took it to an extreme. Years ago an affable local man came to the Sky & Telescope office in Cambridge, Mass., looking for some obscure information. He visited several times and wouldn't take a hint that we were busy. It turned out he believed that he had found evidence in his religion's holy book of how he could unwind all the tangled messes of this world and set everything right. In his apartment he had built a rotating chair centered on a tilted axis aimed at the north celestial pole. He had pictures. He would sit strapped in the chair as it spun opposite the rotation of the Earth. He believed that if just one person did this and unwound all of Earth's daily rotation back to the date of Creation, the whole world would reset itself as good as new. He posted stickers for his website on utility poles.

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