Though the sun gets all the attention August 21, that's only half the story--or at least half the day. Consider the contributions of darkness toward the overall solar eclipse experience.
Look Up That Night
The unmentioned wonder that a solar eclipse begets is a night under a new moon. If the moon is near the sun, then the moon cannot be in the nighttime sky. Think of all the optics that are in the field that day--cameras, telescopes, eyeballs. Turn them all skyward at night! An eclipse by day is the harbinger of darkness by night. Carpe noctem.
Set Your Scope By the Stars
A bevy of optics is about to come out for sungazing. Some telescopes will have mounts with clock drives to track the sun during the eclipse. To set or orient the telescopes best, the owner often aligns the telescope by the stars the previous night. Yes, you can get the tripod or base of the scope generally aligned north-south and level, but automated telescopes then refine the drive mechanism using a three-star fix and software. Go-To telescopes can then simply go to the desired object with the push of a button.
Darren Drake, the In-Resident Astronomer at YMCA Camp Eberhart, regularly aligns his telescopes at night for them to function by day. During the recent 2017 AstroCamp, for example, one such telescope tracked a crescent-shaped Mercury in broad daylight--yes, Mercury, the planet closest to the sun. I don't know if I've ever seen a crescent Mercury midday before, but I did this year at AstroCamp thanks to Darren and the previous night sky.
Nighttime Double Header
Around a solar eclipse you can observe the most night sky in a single day. Sound goofy? Midday on August 21, 2017, during totality you may witness a twilight starfield centered on Leo--the first of two starfields. You may see the brighter stars that are within the constellations you'd normally see in late winter. Jupiter is visible to the left of the sun and Venus to the right. Mars and Mercury are up, too, but fainter.
The night of the 2017 solar eclipse, say, 12 hours later, you see a new moon starfield centered on Aquarius--the second of two starfields in a day. These are the stars you always see at night in late summer. With evening twilight descending, Jupiter (still left of the sun) appears low and follows the sun toward the western horizon. Saturn is south. Distant planets Uranus and Neptune and Pluto are up later, but too faint to see.
The north circumpolar stars are in common for both views, but you can add the two starfields (mid-eclipse and nighttime) to the south to get the total degrees of arc you can see in darkness in one day. How many degrees of starry skies, or how many constellations, is that in one day? Unless you're an astronaut in space, it's probably your personal best. Enjoy the double-header.
How Dark Does It Get At Totality?
I don't know. No one seems to have measured mid-eclipse darkness much in the past. In 2017, however, you can quantify how dark the sky gets along the centerline from your site. Globe at Night is hosting a campaign How Dark Does the Sky Get During a Solar Eclipse? See https://www.globeatnight.org/eclipse-2017/ for an activity guide and finder charts. Dr. Connie Walker writes:
If you are on or near the centerline of the path of totality during the solar eclipse on August 21, 2017, you are able to participate in an activity to observe and record the faintest stars visible as a means of measuring how dark the daytime sky gets. By noticing which major stars and planets are visible during the solar eclipse, comparing them to stars on stellar or “magnitude” charts, and submitting that measurement to the online database, your “measurement” will document darkness levels of a daytime sky during a total solar eclipse, a simple question scientists are still pondering.
The basic steps are to record your time and location; find the faintest object mid-eclipse; match your sky to a magnitude chart; and estimate the cloud cover. Using the Sky Quality Meter or Dark Sky Meter phone app is optional. Thanks for helping to measure the night midday.
Even if you are not in the zone of totality, notice any difference in the daytime as maximum eclipse approaches. For example, as the sun diminishes in size from a disk and nears being a point, shadows become much sharper. Near the edge of totality the sun is still dangerously brilliant though a furtive glance suggests something is amiss overhead. Further afield, no change in darkness may be discernible. You decide.
Night in the Parks
Revisiting the notion about looking up the night of the eclipse...
Among the favored places to watch the eclipse are US National Parks and other national lands. (Please be especially vigilant in preventing wildfires, for firefighters and other first responders are stretched thin this week.) Visitors to each of them should look for scheduled stargazing programs. Or be your own ranger. Get out and look up at the night. Perhaps you'll catch a late shooter from the Perseid meteor shower, which will have peaked the previous week. This may be one of those nights you later remember for the darkness of its night and the multitude of stars.
If you want to support the night sky along Lake Michigan, consider supporting in word or in deed the effort underway to make Indiana Dunes National Lakeshore a designated "Dark Sky Park" recognized by the International Dark-Sky Association (IDA). The park is an enclave away from the light carnage of lakefront cities. Indiana Dunes National Lakeshore hosts regular nighttime observing sessions with telescopes for the public at Kemil Beach. The IDA designation would show leadership in dark sky advocacy for other communities, businesses, and individuals to follow.