This Week’s Sky at a Glance, April 17 – 25

Some daily sky sights among the ever-changing Moon, planets, and stars.

A month after it erupted, the nova in the Sagittarius Teapot continues to fluctuate between about magnitude 4.5 and 6. If it's a "slow nova" it could become even brighter this summer. It's now fairly well up in the south before the beginning of dawn. See article with charts and up-to-date light curve.


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Watch as the Aldebaran-Venus-Pleiades triangle stretches out taller and sideways this week.

Friday, April 17

Every evening Venus continues to stay put at nearly the same spot above the western twilight landscape, while Aldebaran, the Hyades and Pleiades slide farther down to the lower right behind it.

Callisto occults Ganymede among Jupiter's moons. Dark Callisto crosses in front of Ganymede from 9:27 to 9:37 p.m. Eastern Daylight Time. Their combined light will dim by 1.0 magnitude during the middle of this time. Start watching with your scope early to see them moving together into a blend just beforehand.

Saturday, April 18

After dusk, look very high in the west (far to the upper left of Venus) for Pollux and Castor lined up almost horizontally. They're to the lower right of bright Jupiter. Pollux and Castor are the heads of the Gemini twins, now standing almost upright. They form the top of the enormous "Arch of Spring." The Arch's two ends are Procyon to their lower left, and brighter Capella farther to their lower right.

New Moon (exact at 2:57 p.m. EDT).

The waxing crescent Moon poses left of Mercury and Mars very low after sunset, then the Pleiades, then Venus higher up.

The waxing crescent Moon poses left of Mercury and Mars very low after sunset on Sunday the 19th, then the Pleiades on Monday, then Venus on Tuesday higher up. The blue 10° scale as about the width of your fist at arm's length. The Moon is drawn three times its actual apparent size.

Sunday, April 19

Mercury and very faint Mars are near the hairline crescent Moon very low in the west-southwest in bright twilight, as shown at lower right. Bring binoculars.

Monday, April 20

In twilight, look for the sliver of the crescent Moon in the west. It pins the corner of a quadrilateral with Venus, Aldebaran, and the Pleiades, as shown here.

Io eclipses Europa among Jupiter's moons. Io casts its shadow onto Europa this evening from 11:44 to 11:47 p.m. Eastern Daylight Time. Europa will dim by 1.8 magnitudes — a factor of five! — at the middle of these few minutes.

Now that Jupiter is far from opposition, we see shadows in the Jovian system falling far enough sideways that an eclipsed satellite and its eclipser appear widely separated in a telescope's view. So we can see the eclipsed satellite dimming by itself, uncontaminated by the light of the eclipser. (The tables in Sky & Telescope for these events presume that the two satellites appear blended and give their combined magnitude.) See Bob King's article Catch the Last Best Antics of Jupiter’s Moons.

Tuesday, April 21

The waxing crescent Moon shines left of Venus and above Aldebaran in twilight, as shown above.

Far to their lower right, Mercury and faint Mars are within 1.6° of each other this evening and tomorrow evening. Look early with binoculars.

Wednesday, April 22

The Lyrid meteor shower, usually quite weak, should be at its modest peak tonight from about 11 p.m. until the first light of dawn Thursday morning.

Thursday, April 23

It's almost May, but the winter star Sirius still twinkles low in the southwest as twilight fades. How much later into the spring can you keep Sirius in view?

Friday, April 24

The Moon tonight sits on (or near) one side of a big, almost equilateral triangle: bright Jupiter to the Moon's upper left, Pollux upper right of the Moon, and Procyon to the Moon's lower left.

As the Moon waxes past first quarter, it glides under Jupiter and the Sickle of Leo.

As the Moon waxes past first quarter, it glides under Jupiter and the Sickle of Leo.

Saturday, April 25

First-quarter Moon (exact at 7:55 p.m. EDT). Jupiter shines closer to the upper left of the Moon this evening. Jupiter poses directly above the Moon as the night grows late. Although they may look close together, Jupiter is nearly 2,000 times farther away — and 40 times larger in diameter.

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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 an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.


This Week's Planet Roundup

Jupiter on April 1, 2015

On April 1st, Io and its shadow were crossing Jupiter when Christopher Go took this image at 11:17 UT. South is up. The South Equatorial Belt continues to be a little wider and less dark than the North Equatorial Belt. The Red Spot Hollow around the spot is wide and light-colored, and the Red Spot's wake continues to be turbulent. Far below, in the narrow, broken North Temperate Belt, the thin, dark red barge has become more elongated than it was a few weeks ago. These stacked-video images have been sharpened and contrast-boosted.

Jupiter on April 6, 2015, by Christopher Go

How many changes can you find on this image taken five days later, on April 6th?

And again, April 13th.

And again, April 13th.

Mercury (about magnitude –1.4) is emerging from deep in the glow of sunset. About 20 or 30 minutes after sunset, look for it just above the horizon very far to the lower right of Venus. Mercury gets a little higher and easier every day. Mars, much fainter, is in the close vicinity.

Venus (magnitude –4.1, in Taurus) blazes in the west during and after twilight — the brilliant "Evening Star." It doesn't set in the west-northwest until nearly two hours after dark. In a telescope Venus is still small and gibbous, but each week it's growing and thinning as it approaches us in its orbit.

Mars (magnitude +1.4) is deep in the sunset near Mercury — which is more than 10 times brighter. Early this week, Mars is to Mercury's upper left. They appear closest together on April 21st and 22nd, separated by 1.6° or less, with Mars to Mercury's left. Bring binoculars!

Jupiter (magnitude –2.2, in Cancer) shines high in the south as the stars come out, and less high in the southwest after dark. It's the second-brightest point of light in the sky, after Venus. Don't expect to see nearly the detail with your eye that video-frame stacking with large amateur telescopes can bring out, as in the fine images here.

Saturn (magnitude +0.2, just above the head of Scorpius) rises around 10 or 11 p.m. daylight-saving time and is highest in the south in the middle of the early-morning hours. Below or lower left of Saturn (by 9°) is orange Antares, not as bright. The next brightest star in the area is Delta Scorpii, half as far from Saturn. Delta Sco is now in its 15th year of outburst!

Uranus is deep in the glow of dawn.

Neptune is becoming observable quite low (from mid-northern latitudes) at the beginning of dawn. The farther south you are, the higher it will be.

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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 that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 hours.

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“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”

— Neil deGrasse Tyson, 2014.