Scorpius and Sagittarius creep low but bright in summer sky

As night deepens, the moon will rise higher and brighter, and the stars of the constellation Scorpius will glimmer into view around it. The brightest of these is Antares, a fire-orange twinkler - like a spark flown up from a summer campfire.

Look to the upper right of Antares for a not-quite-vertical line of three stars, diamond white. These are supposed to mark the Scorpion’s head. Antares is the creature’s heart, and the rest of him goes looping down to the horizon.

Left of the Scorpion’s lower parts is the next constellation of the zodiac, Sagittarius. Its brightest stars form a teapot shape, as shown here. The Teapot is still pretty low at nightfall, but both it and Scorpius rise somewhat higher later in the night.

These two constellations are strictly sights of summer, at least if you do your sky gazing in the evening. They’re so far south that they pass through New England’s view for only a couple of months of the year, and not very high.

The full moons of June and July always shine in or near these two constellations, so these full moons also ride low across the south, glowing yellow through the thick low-altitude air. If the full moon did not do this, June and July would not be summer.

Here’s how it works. When the moon is full, it’s directly opposite the sun as seen from our viewpoint on Earth. That is, when you face the full moon, the sun is directly behind your head, essentially shining over your shoulder. That’s why you see the moon’s full sunlit face.

In June and July, the sun is the farthest north it ever gets in Earth’s sky, which is why it shines highest and longest every day for those of us in the Northern Hemisphere. Which is what makes it summer. The full moon, being opposite the sun, is farthest south in June and July: in the farthest-south constellations of the zodiac. So the moon rides its lowest across the night.

On the other side of the year in December and January, the situation is reversed. The sun is lowest, and the winter full moon passes about as high overhead around midnight as the summer sun does around noon.

To our great-grandparents on the farm, this was grade-school astronomy that everyone grew up knowing. What we miss nowadays by growing up indoors!

But even through Greater Boston’s light pollution, look back again to that row of three stars marking the Scorpion’s head. The middle of the three is distinctly the brightest. But it wasn’t always so.

Before July 2000, the middle star, named Delta Scorpii, was always a trace fainter than the one above it, Beta Scorpii. Something happened in the course of a month to make Delta nearly double in brightness. It has been fluctuating - but brighter than normal - ever since. Astronomers have yet to figure out exactly what’s going on, despite Delta’s tantalizing, naked-eye obviousness.

Here’s some of what we know. Delta Scorpii is a huge, blue-hot star roughly 14,000 times brighter than the sun, bright enough to be so easily seen even from 400 light-years away. It is spinning so fast that it’s not a sphere, but an ellipsoid. It sheds rings of bright gas from its equator in time with its flare-ups. A lesser star circles it every three weeks, and a third star orbits it on a wildly looping path every 10 years.

The start of the brightening in July 2000 coincided with this third star making one of its close swing-bys. Astronomers are guessing that the tidal effects of the third star saying a close hello triggered the main star’s instability - but how this worked, or why it never happened at the previous 10-year swing-bys, are a mystery. Astronomical detectives remain on the case.

All of which is stuff your great-grandfather never could have known. If we’re becoming personally less connected to the night sky, at least we’re learning a whole lot more about how stuff in it works.

I think this is a good trade, all things considered, but I’m not sure.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.