With the Perseid shower, a great morning apparition of Mercury, and a chance to see craters scratch out an X on the moon, skywatchers have a busy week ahead!
The waxing gibbous Moon may make a mess of this year's Perseids, but I'll still be out there. Even a full Moon can't kill the year's most anticipated meteor shower. Reduce the numbers, yes, but you're guaranteed to see at least a few. I can't wait to ease into that recliner and watch the sparks fly.
The annual Perseids will peak overnight Monday August 12–13 at the same time a glaring gibbous Moon holds forth in Sagittarius about 10° east of the planet Saturn. During my many years of watching Perseids from a semi-rural location (Bortle 4 sky) ] I've usually seen about 40 meteors per hour during a moonless maximum. With the Moon present this time around that number will be cut in half. Most sources give a zenithal hourly rate of 100 per hour, but this is only under ideal circumstances with the shower's radiant at the zenith and stars visible to magnitude 6.5.
2019 will not be kind to either the Perseids or the year's other big shower, the Geminids, when the Moon will be a day past full. Here are a few options for the Perseids. You can go out Monday night the 12th, face away from the Moon so it doesn't further compromise your night vision, and enjoy what comes. The radiant stands some 30° high in the northeastern sky by 11 o'clock, a good time to begin watching. If you have kids, you can go out earlier — there will always be a few to see.
Or you can rise around moonset (~3:30–4:00 a.m. Tuesday morning) shortly before the start of morning twilight. During this brief interval, the shower radiant will be ideally placed high in the northeastern sky and you may see a nice flurry of activity.
You can also benefit from the fact that the Perseids are active from late July to mid-August and avoid the Moon altogether. The shower will be moderately active on the mornings of the 11th and 12th. The high altitude of the radiant combined with zero moonlight should make for a good show at those times.
Observers will have 1-plus hour of darkness on Monday morning August 12th and two hours on Sunday the 11th. Sunday might be ideal in another way. You can further pad your meteor-time by watching a half-hour into morning twilight.
To maximize these slices of dark sky, find your sunrise time and then back up 1 hour and 40 minutes (latitude 37°–42° N) to account for twilight. That's when you'll see the very first hint of dawn. Observers living between 42°–48°N should subtract 2 hours.
Perseids begin life as sand-to-peanut-sized flotsam and jetsam shed by Comet 109P/Swift-Tuttle when it swings near the Sun every 133 years. Earth barrels through this cometary dandruff every late July–early August. Striking the atmosphere around 59 km per second, the particles incandesce while also ionizing the air to create the brief, bright streaks many still call shooting stars. Perseids are swift, white, and often leave glowing trains that persist for a half-second or longer after the meteor disappears. Perseids can appear anywhere in the sky but can be traced backwards to a point in the sky called the radiant, located in western Perseus a short distance below the W of Cassiopeia.
If you choose to observe the shower before dawn, you'll also have the opportunity to see Mercury at the peak of a good morning apparition for skywatchers in the Northern Hemisphere. It shines near –0.5 magnitude on August 13th about 10° below Pollux and Castor in Gemini 45 minutes to an hour before sunrise. Greatest elongation occurs on the 10th when the planet will look like a tiny half-Moon in a telescope. On August 17th Mercury passes slightly less than 1° south of the center of the Beehive Cluster.
Finally, on Wednesday night, August 7th we'll have the opportunity to see the Lunar X, a happenstance pattern of light in the shape of an X created by low-angled sunlight touching the rims of the adjoining craters Blanchinus, Le Caille, and Purbach. It's also known as Werner X (after a nearby crater) or The Purbach Cross. The X slowly creeps into view as the Sun's first rays touch the craters' rims while their interiors are still steeped in deep shadow.
The X is visible around the time of first-quarter Moon every month of the year somewhere on Earth but infrequently from any particular location. If you're hit with cloudy skies or the Moon isn't visible when the X appears, here are several additional times you can look:
* Sept. 6 — 15:47UT (subtract 4 hours for Eastern Daylight-Saving Time, 5 for Central, etc.)
* Oct. 6 — 4:17UT
* Nov. 4 — 17:18UT (DST ends; subtract 5 for Eastern Standard Time, 6 for Central, etc.)
* Dec. 4 — 6:44 UT
Clear and meteor-filled skies!