This Week’s Night Sky: Watch for Auroras Triggered by Sun Stream

Also this week, the thunder moon rises and Neptune does a disappearing act.

Sunspots and Auroras. While solar activity appears to have been calm of late, things may change soon with the appearance of two large sunspot groups now facing Earth.

According to SpaceWeather.com, one of the sunspot groups is associated with an unstable magnetic field, which increases the odds of a solar flare. Already, a stream of high-speed solar wind thrown off the sun this weekend is expected to reach Earth on July 18—exactly the kind of activity that can spark auroras.

Any geomagnetic storm triggered by this event is most likely to be minor, with auroras visible only at high latitudes late on July 18 and 19. Southern Hemisphere sky-watchers with their long winter nights may be best placed for the potential light show.

Thunder Moon. The giant silver orb of the full moon arrives officially at 6:57 p.m. ET (22:57 UTC) on July 19, and it will rise locally in the eastern sky just as the sun sets in the west. The moon will appear to straddle the borders of two constellations: Capricornus, the sea goat, and Sagittarius, the archer.

The July full moon has taken on many traditional names in North America, according to the Old Farmer’s Almanac. It’s sometimes called the thunder moon, in reference to the frequent stormy weather seen in July, or the hay moon due to the common practice of cutting hay this time of the year. The most common moniker, however, is the buck moon, because this is when male deer begin showing signs that they are growing their antlers.

Look for the moon to guide binocular and telescope users to the faint bluish disk of Neptune on July 22. [SKYCHART BY ANDREW FAZEKAS, SKYSAFARI]

Neptune and Luna. As darkness falls on July 22, look for the waning gibbous moon to be parked very close to the most distant planet in the solar system, Neptune. The ice giant is so far away that it takes 165 Earth years to complete on trip around the sun. Shining at a feeble eighth magnitude, Neptune’s tiny blue-green disk will really be visible only through binoculars and backyard telescopes.

Lucky observers in eastern North America will get a bonus show late in the evening, when Neptune disappears behind the lighted side of the moon and reappears along its darker limb. The exact times of this lunar occultation event will vary depending on your location, so get out early to make sure you catch the show.

Six bright stars mark the constellation Cygnus, the swan. The bird's head is formed by the double star system Albireo. [SKYCHART BY ANDREW FAZEKAS, SKYSAFARI]

Double Gem. The evening of Sunday, July 24 will offer a great chance to check out a very pretty, double star system. Observers across the Northern Hemisphere looking overhead on late nights can easily see the bright constellation Cygnus, the swan, otherwise known as the Northern Cross.

The constellation has six bright stars that form a distinctive, lopsided X-pattern easily seen even from light-polluted suburbs. The brightest of the bunch is blue-white Deneb, marking the swan’s tail. On the other side, orange-hued Albireo marks the bird’s head.

It’s definitely worth taking a gander at Albireo, a dying star some 700 times brighter than our sun that sits about 385 light-years away. Binoculars easily reveal this elderly giant to be a magnificent double star. The fainter companion is a sapphire-colored gem that lies more than 392 billion miles (630 billion kilometers) away from its ruddy neighbor.

Clear skies!

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