FRIDAY, APRIL 7
■ Mercury is now in easy view in late twilight far to the lower right of Venus, as indicated in the scene below.
And once the sky is dark enough, spot the Pleiades above Venus. They're closing in on each other!
■ Vega, the bright "Summer Star," rises in the northeast within an hour after the end of twilight, depending on your latitude.
Exactly where should you watch for Vega to come up? Spot the Big Dipper very high 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 makes its appearance.
SATURDAY, APRIL 8
■ Castor and Pollux shine together high toward the west-southwest after dark. Pollux, on the left, is slightly the brighter of these "twins." Draw a line from Castor through Pollux, follow it farther left by a big 26° (about 2½ fist-widths at arm's length), and you're at the dim head of Hydra, the Sea Serpent. In a dark sky it's a subtle but distinctive star grouping, about the width of your thumb at arm's length. Binoculars show its stars easily through light pollution or moonlight.
Continue the line farther by a fist and a half and you hit Alphard, Hydra's orange heart.
Another way to find the head of Hydra: It's almost midway from Procyon to Regulus.
SUNDAY, APRIL 9
■ The waning gibbous Moon rises around midnight tonight, closely accompanied by orange Antares. By the time dawn begins Monday morning, they're well up in the south.
■ The huge, bright Winter Hexagon is still in view early after dark, filling the sky to the southwest and west. It's the sky's biggest widely-recognized asterism.
Start with brilliant Sirius in the southwest, 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), right or lower right from Castor to dim Beta Aurigae and then bright Capella, lower left from there to Aldebaran, lower left to Rigel at the bottom of Orion, and back to Sirius.
The Hexagon is distended. But if you draw a line through its middle from Capella to Sirius, it's fairly symmetric with respect to that axis.
MONDAY, APRIL 10
■ Venus is passing 2° left of the Pleiades this evening and tomorrow evening, as shown below. How soon before the end of twilight can you first begin to see the little cluster?
Of course they're nowhere near each other, really. Venus this evening is 9.4 light-minutes from us, while the Pleiades are 440 light-years in the background. That's 25 million times farther away. To put this in scale-model perspective: If Venus were a shiny dust speck floating five inches in front of your eye, the Pleiades stars would be 1,200 miles behind it halfway across the continent: blue-white-hot marbles and peas, searingly brilliant, scattered in a volume of black space about 30 miles wide.
Also in and around this volume would be many fainter Pleiades stars: scores of dimmer, yellow-hot BBs (Sun-like stars), orange-hot mustard seeds (orange dwarfs), and roughly a thousand much dimmer, merely red-hot sand grains: red and brown dwarfs.
And on average, each one of these would be a few miles from its nearest neighbor. Space is big, even in a star cluster.
TUESDAY, APRIL 11
■ Right after dark, Orion is still well up in the southwest in his spring orientation: striding down toward the right with his belt horizontal. The Winter Hexagon fences him in.
His belt points left toward Sirius and right toward Aldebaran and, farther on, Venus and the Pleiades.
WEDNESDAY, APRIL 12
■ The last-quarter Moon rises as late as about 3 a.m. daylight-saving time tonight, depending on your location. (The Moon is exactly last quarter at 5:11 a.m. Thursday morning EDT.) Before the first light of dawn begins on Thursday, look upper right of the Moon for the Sagittarius Teapot sitting almost level.
THURSDAY, APRIL 13
■ This is the time of year when, as the very last of twilight fades away, the bowl of the Little Dipper extends straight to the right of Polaris. Most of the Little Dipper is dim, but Kochab at the lip of its bowl is Polaris's equal at 2nd magnitude. Spot Kochab 16° to Polaris's right: about a fist and a half at arm's length.
Look high above Kochab and the other end star of the Little Dipper's bowl (Pherkad), you'll find the two end stars of the Big Dipper's bowl, the Pointers.
FRIDAY, APRIL 14
■ Mars this evening shines just a fraction of a degree from Epsilon Geminorum, a fifth as bright. Binoculars give a fine view. See "This Week's Planet Roundup" below.
SATURDAY, APRIL 15
■ In early dawn Sunday morning, spot the waning crescent Moon very low in the east-southeast. About 5° above it is distant Saturn, beginning its nearly year-long apparition.
SUNDAY, APRIL 16
■ Right after dark, the Sickle of Leo stands vertical high in the south. Its bottom star is Regulus, Leo's brightest. Leo himself is walking horizontally westward. The Sickle forms his front leg, chest, mane, and part of his head. Off to the left, a long right triangle forms his hind end and long tail.
This Week's Planet Roundup
Mercury is having a nice evening apparition in the west, but catch it while you can. In twilight, look for it roughly two fists lower right of bright Venus. Mercury is fading fast now: from magnitude –0.4 on Friday April 7th to +0.6 a week later. That's a two-thirds loss of brightness in one week!
(And why is that much of a magnitude drop a fade by two thirds? See the bottom of this page for the answer.)
Venus (magnitude –4.1, in Taurus) is the brilliant "Evening Star" shining in the west during and after dusk. It doesn't set until two hours after dark.
Venus visits the Pleiades this week, passing 2° left of the cluster on April 10th and 11th. Telescopically, Venus is a shimmering little gibbous ball 15 arcseconds in diameter and about 74% sunlit.
Mars is crossing the Castor stick-figure in Gemini. Look for it high in the west in early evening, lower in the west later. It's upper left of Venus by three or four fists at arm's length.
Mars continues to fade, from magnitude +1.1 to +1.2 this week. That makes it equal to modest Pollux above it (mag +1.15), although Mars shows a deeper orange tint. Mars is nearly on the far side of its orbit from us, so in a telescope it's just a tiny blob 6 arcseconds wide.
Jupiter is out of sight, in conjunction with the Sun.
Saturn, magnitude +1.0 in dim Aquarius, is emerging into early-dawn view very low in the east-southeast. Binoculars help. Look for it far, far below Altair.
Uranus, magnitude 5.8 in Aries, is lost from view in twilight, positioned between Venus and Mercury. (But it's still gloriously clear to the James Webb Space Telescope!)
Neptune, mag 8.0, is hidden in the sunrise.
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 sometimes called UTC, GMT, or Z time.
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.
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), which shows all 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. 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."
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.
"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
1 . Okay, why does Mercury fading by 1.2 magnitudes mean it fades by two thirds? How do you convert 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.
So, 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: