Although there are plenty of fascinating objects to observe in June skies, night time astronomy becomes increasingly inconvenient in the northern hemisphere as we enter the month of June. On June 10, the sun rose at 5:18 this morning and set at 8:52 p.m. And then, of course, there is twilight.
Twilight is a general brightening of the sky while the sun is near but below the horizon. “Civil twilight” — meant to describe the duration of twilight as noticed by a casual observer — occurs while the sun is less than 6 degrees below the horizon (a bit more than one fist width at arms length). At the end of civil twilight brighter stars are easily seen, and some moderately bright stars are visible (in locations outside of cities). But even after civil twilight has ended, sunlight still affects the brightness of the sky at a level that interferes with our ability to see dim objects through the telescope until astronomical twilight ends - when the sun is 18 degrees below the horizon.
Because the angle at which the sun travels when it sets is shallowest near the first day of summer (and again near the first day of winter), twilight is longest at this time of the year. Civil twilight on June 10 ended at 8:59 p.m. and morning twilight began at 4:42 a.m. June 11 — giving us only 7 hours and 43 minutes of “darkness." Complete darkness, as limited by astronomical twilight, will only last about 4 and a half hours tonight.
Every day in the northern hemisphere, the Sun draws an arc through the sky from morning to night, reaching its highest point above the horizon when it lies directly south of us. The old nautical term for when an astronomical object reaches its highest point above the horizon is therefore “southing."
As we approach June 20th, the first day of summer, every day the height of the Sun in the sky at the moment of southing is higher than the day before — and the arc described by the Sun throughout the day is longer. This lengthening of the day is directly caused by the positioning of the Earth and Sun at the beginning of summer. The Earth’s tilted axis — the line about which the Earth rotates daily – is aligned on June 20th so that the northern tip of the axis (the north pole) leans in the direction of the Sun.
On June 20th, the sun will reach its highest point over the horizon for the year. The Sun (Sol in Latin) will stop (stice) its increase in height on that day, and every day thereafter, until December 21st, the sun will south lower in the sky than the day before. The exact date of the solstice changes from year to year because the Earth’s orbit is not completed in precisely 365 days. We correct for most of this error using leap years, and there is a much slower drift in the dates of the solstices, but those are topics for another time.
Rather than fight the Sun by staying up through the late evening into morning, why not take advantage of the long days — and observe the Sun? As you have heard in earlier posts, you must never try to look directly at the Sun, and absolutely never even think of observing the sun by looking directly through a telescope or binoculars, as the result would be instant and permanent blindness.
You can safely view the Sun by projecting its image onto a white card or sheet of paper, using either a telescope or binocular to create the image. Using a telescope or binocular, you need to point the scope in such a way that creates the smallest shadow. You are then very close to aiming at the Sun, and moving it around slightly with the card held a few inches behind the eyepiece (and looking on the side of the card facing the eyepiece), you will soon see a bright image fall on the card. You then will want to focus the scope or binocular to get a sharply defined circle of the Sun.
Now take a careful look in the central region of the Sun, and look for small gray or black spots. These are not dirt (unless you have dirt on the paper screen J), but sunspots. This is a perfect time to go sunspot hunting, as spots will be very prevalent on the Sun for the next year to two years. [A pinhole camera, such as you may use to safely observe a solar eclipse, is not capable of giving a sharp enough image to be useful in finding sunspots].
Though we think of the Sun as a constant and unchanging object, it indeed undergoes changes in its activity over a roughly 11 year cycle. The most prominent changes are in the number of sunspots occurring on its surface, and the number and intensity of solar flares – mass ejections of plasma from the Sun which can interfere with radio and other forms of communication here on Earth. We are now near a peak in the solar cycle, with the actual peak expected to come next year.
In the photos included with this post I have several images that I’ve taken over the past two weeks of sunspot groups that would be easily seen using the smallest of telescopes or binoculars. These photos are highly magnified, what you would see would look more like specks of pepper for the smallest spots, or black pimples in the case of the large spots.
A great deal more can be said on the topics of the summer solstice, sunspots, and the solar cycle than I can fit in this blog post. I’ll save these discussions for future blogs, but you may also want to check out the classes I offer for school-aged children, at www.turnerclasses.com. The next sessions are starting in August, and I would definitely like to see you there!