As we write this, the total solar eclipse that will engulf St. Joseph, Missouri, is still more than two and one-half years away. People, however, are beginning to get nervous. Hey, that’s what people do! They start asking themselves questions: “How should I prepare?” “What do I need to know about eclipses?” “Where should I go?” Front Page Science has designed this website to answer those questions — and many more. Come to Rosecrans Memorial Airport. We have it all figured out.
I have, however, encountered people who desire a bit more than a website, no matter how much information it contains. They want to get out under the sky and check out the circumstances themselves. If you’re one of these needy souls, We have good news: There is a way to conduct a rehearsal for the eclipse.
First, some astronomy background. Our planet’s axis tilts 23.5° to the plane of its orbit around the Sun. This orientation explains why we experience seasons. When the northern tip of Earth’s axis points toward the Sun, it’s the Northern Hemisphere’s summer. When the southern tip of the axis points to the Sun, the season in the Northern Hemisphere is winter. Spring and autumn lie midway between those extremes. All seasons reverse in the Southern Hemisphere.
Because of the tilt, the Sun’s altitude at any location changes by 47° in a six-month span. On the June solstice (the first day of summer in the Northern Hemisphere), the Sun stands as high in the sky as it will be all year. Conversely, on the December solstice (the first day of winter in the Northern Hemisphere), the Sun is as low in the sky as it gets all year.
When astronomers give the Sun’s position, they use the two celestial coordinates right ascension and declination. These two values roughly correspond to latitude and longitude on Earth. The Sun’s declination varies from most southerly (December solstice) to most northerly (June solstice). Except at those extremes, then, the Sun will have the same declination twice during the year.
On August 21, 2017, the Sun’s declination will be 11° 51′, approximately. In 2015, the date when the Sun’s declination is closest to that value is August 22. So, if you want to “practice” observing the Sun where it will be on eclipse day, head out August 22. Maybe you want to set up a filtered telescope. Maybe you want to take a few pix (although we do not suggest photographing the eclipse, especially if this will be your first). Maybe you just want to check out a prospective observing site. How far away are any trees? Buildings? Whatever, you can see how the Sun will perform on eclipse day.
Recall that we showed there were two dates where the Sun had the same declination during the year. The other date in 2015 where our star’s declination comes closest to 11° 51′ is April 21. On that date, the Sun’s path through the sky will be the same as it will be on eclipse day in 2017. So, anything you want to try on eclipse day you can practice April 21 or August 22
As you might have guessed, 2016 also will have two dates suitable for a rehearsal: April 20 and August 21. And during the following year, April 20, 2017 also will work.
The dates we list above are the closest approximations to what you’ll see on eclipse day. The Sun will rise and set around the same times, and it will cross the meridian (the imaginary north-south line that passes through the overhead point; the Sun crosses it at midday) the same time as on eclipse day. Here’s something to consider, however. The Sun’s declination doesn’t change all that much from day to day. In fact, if your rehearsal occurs as many as three days before or after any of these listed dates, you really won’t notice any difference when August 21, 2017, rolls around.
For your convenience, we have arranged the dates into a table: