This Week's Sky at a Glance, April 22 – 30

FRIDAY, APRIL 22

■ Arcturus is the brightest star high in the east these evenings, shining pale yellow-orange. Look for Spica, paler blue-white, lower right of Arcturus by about three fists at arm's length.

To the right of Spica by half that distance is the distinctive four-star constellation of Corvus, the springtime Crow.

■ The Lyrid meteor shower may still be active late tonight, although the predicted peak of the shower is not timed well for North America. Best time to try watching is from about 11 p.m. until moonrise, as Bob King writes in Celebrate Spring with the Lyrids. The shower is usually pretty weak. Under good observing conditions you might see a Lyrid every 5 minutes or so of steady watching, on average.

■ The last-quarter Moon rises as late as 3 a.m. daylight-saving time Saturday morning. Watch for it to come up far to the lower right of Altair. The Moon (in Capricornus) is exactly last-quarter at 7:56 a.m. Saturday morning EDT.

SATURDAY, APRIL 23

■ Vega, the Summer Star, the zero-magnitude equal of Arcturus, twinkles low in the northeast after nightfall. . . depending on your latitude. The farther north you are the higher it will be. If you're in the latitudes of the southern US, you'll have to wait until a bit later after dark for it to appear.

■ High up above Vega, the Big Dipper is tipping over. The middle star of the Dipper's handle is Mizar. A line from Mizar through its close little companion Alcor (binoculars help!) points the way down to Vega. Or to the place on the horizon where it will soon rise.

■ From Sunday through Wednesday mornings, the waning Moon passes under Saturn, Mars, and the Venus-Jupiter pair low in the southeast, as shown below.

SUNDAY, APRIL 24

■ Look west-southwest as the stars come out. There's Orion tilting down low in his spring orientation, with his belt horizontal. The belt points left toward Sirius and right toward Aldebaran and, farther on, the Pleiades. They'll all soon set.

■ Look very high above Orion for Pollux and Castor, lined up roughly horizontally. They form the top of the huge Arch of Spring.

Lower left of Pollux and Castor, Procyon forms the Arch's left end. Look farther to the lower right from Pollux and Castor for 2nd-magnitude Menkalinan and then brilliant Capella, the Arch's right end.

The Arch is the eastern half of the even larger Winter Hexagon, the rest of which is getting low or setting.

MONDAY, APRIL 25

■ Leo walks horizontally across the meridian these evenings. The Sickle of Leo (the Lion's head, front, and forefoot Regulus) stands upright, with its open side to the right: a backward question mark. Its brightest stars are Regulus, the bottom of the Sickle's handle, and Gamma Leonis (Algieba), above Regulus in the Sickle's crook.

About two and a half fists left of Regulus is Denebola, Leo's tail-tip.

TUESDAY, APRIL 26

■ In late April, the two Dog Stars align vertically as the stars come out in mid-twilight. Look southwest. Brilliant Sirius in Canis Major is rather low. Procyon in Canis Minor stands above it by 25°, two or three fists at arm's length.

WEDNESDAY, APRIL 27

■ Arcturus is climbing high in the east, brilliant at magnitude zero and pale yellow-orange. Next, turn around to the northwest. Descending there is equally bright Capella. They stand at exactly the same height above your horizon in late twilight as the stars are coming out, depending on both your latitude and longitude.

How accurately can you time this event for your location? Like everything constellation-related, you'll find that it happens 4 minutes earlier each night.

THURSDAY, APRIL 28

■ These spring evenings, the long, dim sea serpent Hydra snakes far across the southern sky. Find his head, a rather dim asterism about the width of your thumb at arm's length, in the southwest. It's lower right of Regulus by about two fists at arm's length. Also, a line from Castor through Pollux points to it about 2½ fists away. Hydra's tail stretches all the way to Libra rising in the southeast. Hydra's star pattern, from forehead to tail-tip, is 95° long.

FRIDAY, APRIL 29

■ Venus-Jupiter conjunction. Set your alarm to get up and look low in the southeast about 60 to 45 minutes before your sunrise time Saturday morning. Venus and Jupiter, the two brightest objects after the Sun and Moon, are a spectacularly close ½° apart!

On Sunday morning they'll be only a trace farther apart, with Jupiter now to Venus's upper right.

SATURDAY, APRIL 30

■ Face north just after nightfall, look very high, and you'll find the Pointers, the end stars of the Big Dipper's bowl, on the meridian pointing toward Polaris straight down below. From the Pointers to Polaris is about three fists at arm's length.

Whenever the Pointers point straight down, Vega is rising low in the northeast, Leo walks horizontally high across the south, and the Arch of Spring (see Sunday above) fills the high west.

■ A partial eclipse of the Sun occurs today for the southeastern Pacific and the southern cone of South America. Details.

 

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This Week's Planet Roundup

Mercury is having its best evening apparition of the year right now. As dusk dims, look for it low in the west-northwest about 45 to 60 minutes after sunset. But watch as Mercury fades rapidly this week, from magnitude –0.5 on Friday the 22nd to +0.3 on the 29th. That means it loses half its light. ¹

Venus and Jupiter, magnitudes –4.2 and –2.1, respectively, are the two "Morning Stars" shining low in the east-southeast as dawn brightens. They're closing in on each other fast. On the morning of April 23rd, Jupiter is 7° lower left of Venus. On the mornings of April 30th and May 1st, they're ½° apart at the times of dawn for the Americas.

Their actual conjunction happens around 19 hours UT April 30th, when the two planets will be ¼° apart. Around that time, they will be in dawn view for the western Pacific Rim.

Mars and Saturn glimmer to the upper right of Jupiter and Venus, in that order. They're vastly fainter, identical now at magnitude +0.9. Mars, however, is more orange than pale yellow Saturn.

Each morning they're a little farther from Venus and from each other. The Venus-Mars-Saturn line is 25° long on the morning of April 23rd and 33° long by the 30th. The three stay about equally spaced from each other.

Uranus is hidden behind the glare of the Sun.

Neptune is in the background of Venus and Jupiter, but at magnitude 7.9 it's unobservable in the dawn glow.

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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. (Universal Time is also called UT, 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 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 magazine of astronomy.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to 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.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And 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."

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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 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."
            — John Adams, 1770

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1 . Okay, you've asked how we do that. Why does Mercury fading 0.75 magnitude mean it fades by half? How do you change a magnitude difference to a brightness difference?

Since 1856, the stellar magnitude system has been precisely defined so that 5 magnitudes is exactly a 100-times difference in brightness. So, one magnitude is a change in brightness of the fifth root of 100. Which is 2.512 for all practical purposes.

Here's the formula to use: If △m is the magnitude difference, then

brightness difference = 2.512^△m

...which is just a few taps on your scientific calculator.

Some amateurs learn the basic magnitude intervals by heart:

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