Sunday, June 4, 2023


This Week’s Sky at a Glance :

The waxing gibbous Moon this evening shines high in Gemini. When you face south after dark, Pollux and fainter Castor shine to the Moon’s left by about a fist at arm’s length, as shown below.

At roughly right angles to that direction, the Moon shines almost exactly midway between Capella and Procyon (during evening in North America): about 2½ fists from each.

The waxing gibbous Moon passes under the heads of Gemini. (The “8 p.m.” is standard time; on Sunday the 13th that means 9 p.m. daylight time.)


■ Pollux and Castor accompany the Moon across the sky tonight, as shown above. Pollux is the one closest to it.

Down below this group is Procyon. Lower right of Procyon shines brighter Sirius.

■ Daylight-saving time begins at 2 a.m. tonight for most of North America. Clocks spring forward an hour.


■ Sirius blazes high in the south on the meridian in late twilight this week. The seeing often steadies in twilight, so this may be a good time to try for Sirius B, the Dog Star’s notoriously difficult white-dwarf companion.

Sirius A and B are at their farthest apart in their 50-year orbit, separated by 11.3 arcseconds (they’re exactly farthest apart next year if you’re picky). They’ll remain very nearly as wide for the next few years before they start closing again.

You’ll want at least an 8- or 10-inch scope and a night of really excellent seeing. Keep checking night after night; top-notch steady seeing makes all the difference for spotting Sirius B. See the additional tips in Bob King’s article Sirius B – A New Pup in My Life.

The Pup is east-northeast of the Dog Star and 10 magnitudes fainter: one ten-thousandth as bright. As Bob recommends, put a homemade occulting bar across your eyepiece’s field stop: a tiny strip of aluminum foil held to the field stop with a bit of tape, with one edge of the foil crossing the center of the field. Hide blinding Sirius A just behind the strip’s eastern edge.

■ A different Sirius challenge: Find it in the daytime! Venus is sometimes not hard to see with the naked eye in a clean blue sky, if you land on the precise spot to examine, and it’s much easier with binoculars or a telescope. Jupiter is tougher. . . but what about the brightest star? At this time of year Sirius is nearing the meridian in late afternoon before sunset. The article in the March Sky & Telescope, page 48, will enable you to give it your very best shot.


■ Spring doesn’t arrive until next week. But the Spring Star Arcturus rises above the east-northeast horizon around the end of twilight now, depending on your latitude.

To see where to watch for it to come up, find the Big Dipper as soon as enough stars come out. It’s high in the northeast. Follow the curve of its handle down and around to the lower right by a little more than a Dipper-length. That’s the spot on the horizon to watch. The farther north you are, earlier Arcturus rises.

By 9 or 10 p.m. Arcturus is high up and dominates the eastern sky.

■ More about Sirius and its constellation Canis Major. In a very dark sky the Big Dog’s realistic stick figure is plain to see — he’s seen in profile prancing to the right on his hind legs, with Sirius on his chest as his shiny dog tag. But for most of us, only his five brightest stars show well through the light pollution. These form the unmistakable Meat Cleaver.

Sirius and Murzim (to its right) are the wide top end of the Cleaver, with Sirius sparkling on its top back corner. Down to Sirius’s lower left is the Cleaver’s other end including its short handle, formed by the triangle of Adhara, Wezen, and Aludra. The Cleaver is chopping toward the lower right.


The nearly full Moon occults Eta Leonis on the evening of March 15th, but only for some locations. The scene here may not quite be yours. Also, in these scenes the Moon is shown three times its actual apparent size.

■ The bright waxing gibbous Moon shines in the Sickle of Leo as darkness falls. The Moon is roughly between Regulus lower right of it and Algieba, slightly fainter, left of it (for North America).

Use binoculars to spot Eta Leonis, magnitude 3.5, very close by the Moon. . . maybe! The Moon occults Eta Leonis for eastern North America. Use a telescope to watch as the Moon’s narrow dark limb approaches the star and then snaps it out. The time for this depends on where you are; for New York, for instance, the occultation happens at 7:51 p.m. EDT.

Map and timetables. Note that the timetable has two long halves: first for the star’s disappearance, then (scroll down) for its reappearance from behind the Moon’s bright limb up to an hour or more later.


■ Pollux and Castor in Gemini pass nearly overhead around 8 p.m. this week if you live in the world’s mid-northern latitudes. They go smack overhead as seen from near latitude 30° north: Austin, Houston, and the US Gulf Coast, as well as northernmost Africa, Tibet, and Shanghai.

The “twin” heads of the Gemini figures are fraternal twins at best. Pollux is visibly brighter than Castor and pale orange. And as for their physical nature, they’re not even the same species.

Pollux is a single orange giant. Castor is a binary pair of two much smaller, hotter, white main-sequence stars, a fine double in amateur telescopes. If Pollux were a basketball, Castor A and B would be a tennis ball and a baseball about a half mile apart.

Moreover, each Castor star is closely orbited by an unseen red dwarf — a marble in our scale model just a foot or so from each of the two bright primaries.

And a very distant tight pair of red dwarfs, Castor C, is visible in amateur scopes as a single, 10th-magnitude speck 70 arcseconds south-southeast of the main pair. In our scale model, they would be a pair of marbles about 3 inches apart at least 10 miles from Castor A and B. Space is big.

■ At nightfall, the Big Dipper is now as high in the northeast as Cassiopeia is in the northwest. The Dipper dominates on spring and summer evenings, and indeed the season is about to change: the spring equinox comes on March 20th.

Unaffected midway between the Dipper and Cas is, as always, Polaris.


■ Full Moon (exactly so at 2:18 a.m. Friday morning Eastern Standard Time; 11:18 p.m. Thursday PST). Once the stars come out look for Leo, the stick-figure lion, stepping upward above the Moon. Leo’s tail-tip, 2nd-magnitude Denebola, is nearly a fist at arm’s length to the Moon’s upper left this evening (for North America).


■ The Moon is barely past full. Once it’s well up late this evening, it forms a nearly right triangle with Arcturus about three fists to its left and Spica half that distance below it (for North America).

In the dawn, Venus, Mars, and Saturn still form their triangle. But the triangle is shortening as Saturn approaches Venus.


■ The bright waning gibbous Moon rises around twilight’s end. Once it’s well up. look for Spica about 4° to its lower right (for North America), glimmering through the moonlight.

Brighter Arcturus shines about three fists to their upper left.


Mercury, Jupiter, and Neptune are out of sight in the glare of the Sun.

Venus, magnitude –4.6, is the bright “Morning Star” shining low in the southeast during dawn.

Mars, vastly fainter at magnitude +1.2, continues to hang 4° lower right of Venus all week. Look for it early before dawn gets too bright; Mars currently shines with a little less than 1% of Venus’s light.

Challenge question! Why is Mars so much dimmer than Venus now? Four reasons combine to make it so, despite two other factors working in the opposite direction. Points for each of the six that you get! Answers at the bottom of this page.

Saturn, magnitude +0.8, is emerging into dawn view lower left of Venus as shown above. Bring binoculars. Their separation closes from 15° on the morning of March 12th to 9° on the 19th.

Saturn will pass 2° under Venus at their conjunction on the mornings of March 28th and 29th, with Mars still nearby.

Uranus (magnitude 5.8, in Aries) is in the west right after dark, less high now.

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 Standard Time, EST, is Universal Time minus 5 hours. Eastern Daylight Time, EDT, is UT minus 4 hours.

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, 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.

  • Answers to why Mars appears so much fainter than Venus. It’s a combination of six factors:

– Mars is a smaller planet. It’s only a little more than half the physical diameter of Venus, which means it has not much more than 1/4 the surface area to reflect sunlight with.

– Mars is much less reflective (has a lower albedo). The dusty brown Martian surface reflects only 15% of the sunlight that falls on it, while Venus’s white cloudtops reflect 65% of incident sunlight.

– Mars is currently twice as far from the Sun as Venus is. So the sunlight that strikes its it only one fourth as intense as sunlight at Venus.

– And, Mars is currently 3 times farther from Earth than Venus is. Three times farther means 9 times fainter.

On the other hand, Venus suffers at present from two effects of its phase:

– First, its globe currently appears less than half illuminated (43% sunlit) as seen from Earth, while Mars is 93% sunlit: gibbous close to full.

– Second, the side of Venus we see is therefore more or less grazingly sunlit, while the nearly full face of Mars is hit by light coming almost face-on.



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