The sky, July 8-14: First quarter Moon occults Spica

Special to, July 8, 2024

Excerpts from weekly Sky&Telescope report.


■ Hercules crosses the zenith these evenings. And in the western edge of the Hercules Keystone, how many times have you pointed a scope at old friend M13, the great globular cluster? You do that not just because it’s one of the finest globulars in the sky, but because it’s easy to find.

And because we fall into habits.

So the other grand globular of Hercules, M92 just 9° away, goes way overlooked. The two seem like, if not twins, close siblings. M92 is nearly as large and nearly as bright as M13, but it’s off in the wilds north of the Keystone.

And what about the ignored little kid of the Hercules globular family, NGC 6229? It’s a few degrees farther north of the Keystone and only magnitude 9.4, but still within reach of smallish scopes. I’ve always seen globulars as sugar piles in moonlight. Don’t wait; the actual Moon will start lighting the evenings in a few days.


■ Soon after nightfall, look due south for orange Antares on the meridian. Around and upper right of Antares are the other, whiter stars forming the distinctive pattern of upper Scorpius. The rest of the Scorpion runs down from Antares toward the horizon, then left.

■ Three doubles in the top of Scorpius. The “head of Scorpius” is the near-vertical row of three stars upper right of Antares. The top star of the row is Beta Scorpii or Graffias: a fine double star for telescopes, separation 13 arcseconds, magnitudes 2.8 and 5.0.

Just 1° below Beta is the fainter, very wide naked-eye pair Omega1 and Omega2 Scorpii, tilted to the right of vertical. They’re 4th magnitude and ¼° apart. Binoculars show their slight color difference; they’re spectral types B9 and G2.

Upper left of Beta by 1.6° is Nu Scorpii, separation 41 arcseconds, magnitudes 3.8 and 6.5. In fact this is a telescopic triple. High power in very good seeing reveals Nu’s brighter component itself to be a close binary, separation 2 arcseconds, magnitudes 4.0 and 5.3, aligned almost north-south.


■ To the right of Antares is that roughly vertical row of Beta, Delta, and fainter Pi Scorpii. The middle one, Delta Sco, is the brightest  obviously so. But it didn’t used to be. It used to be like Beta.

Antares, Delta Scorpii, and Head of Scorpius

The brightest star near Antares is Delta Scorpii, now in its 24th year of an unexpected flareup. Photographs don’t show the brightness differences between stars well, but to the naked eye Delta is obviously the brightest star in the nearly vertical row of three forming the head of Scorpius. Outdoors at night, estimate Delta’s magnitude by comparing it with Beta, magnitude 2.6, and Antares, 1.1.

Delta is a strange variable star, a fast-rotating blue subgiant throwing off luminous gas from its equator. Assumed for centuries to be stable, Delta doubled in brightness unexpectedly in the summer of 2000, then dipped down and up again several times from 2005 to 2010, and has remained essentially steady at peak brightness (magnitude 1.7) ever since. If anything, it has gained just under a tenth of a magnitude in these last 14 years.

Delta has a smaller orbiting companion star that was suspected to trigger activity in it at 10.5-year intervals. Astronomers predicted the system might have another flareup around 2022, when the companion star made its third pass by the primary since 2000. But nothing happened.

No one knows what might happen next, or when.


■ As summer progresses, bright Arcturus moves down the western side of the evening sky. Its pale ginger-ale tint always helps identify it.

Moon passing Spica at dusk, July 12-14, 2024


■ The Moon, just short of first quarter, shines west of Spica this evening as shown below, creeping toward Spica to occult it tomorrow. Plan ahead for your occultation watch!

The Moon steps smack across Spica this month as seen from North America.

(These scenes are always drawn for an observer at latitude 40° N, longitude 90° W, near the population center of the continent. The Moon’s position with respect to Spica will differ a bit depending on where you are. The Moon below is drawn about three times its actual apparent size.)


■ The first-quarter Moon occults Spica this evening for virtually all of North and Central America. The 1st-magnitude star will vanish behind the Moon’s dark limb, then will reappear from behind the bright limb up to an hour or more later. Rarely do we see the Moon occult such a bright star!

Paths of Spica behind the Moon during occultation 7-13-2024

Spica takes a different path behind the Moon depending upon your location. Five cities are shown here. Except for New York, NY and other cities in the Eastern Time Zone, both the disappearance and reappearance of Spica will be visible. Paths are approximate. Map by Bob King with Stellarium

For the East Coast the Moon will be getting low in the western sky (plan a good observing spot beforehand). For much of the West the Sun will still be up — but Spica will probably be visible through the blue sky in a telescope, just not so easily.

In a dark sky even your naked eyes might be enough to catch the disappearance, depending on your vision and the clarity of the air.


■ The tail of Scorpius is at its best low due south soon after dark. It’s about a fist and a half at arm’s length lower left of Antares.

Look for the two stars especially close together in the tail. These are Lambda and fainter Upsilon Scorpii, known as the Cat’s Eyes. They’re canted at an angle; the cat has a bleary eye and is tilting his head to the right. Lambda is brighter than Upsilon; they’re magnitudes 1.6 and 2.6. Both are blue-white supergiants, 700 and 500 light years away, respectively. Yes, the nearer one is the fainter one.

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