By Bill Pellerin
Houston Astronomical Society
Just after midnight (Central Daylight Time) on June 21, summer began officially. You would be forgiven (in some parts of the country) if you thought that summer was here prior to that date, with some especially hot days. June 21 was the summer solstice, the day on which the Sun is the highest in the sky and the longest sunlit day in the year. At the solstice the Sun is at its northernmost excursion.
I live in Houston, Tx, where the latitude is 29.8 degrees north. Does the Sun ever get directly overhead here? No, it doesn’t. That is, the Sun does not reach the zenith. You’re probably aware of two lines on the world map called the ‘Tropic of Cancer’ and the ‘Tropic of Capricorn’. These two lines represent the northernmost and southernmost positions of the Sun at summer solstice and winter solstice. Does the Sun get to the zenith at any point in the continental United States. Again the answer is no.
The northernmost position of the Sun this year is 23 degrees, 26 minutes. So, in my hometown, the Sun was about 6 degrees south of the zenith on June 21 as it crossed the meridian, and close to M35 at the foot of Gemini. How about other locations? Brownsville, Tx has a latitude of 25.9 degrees; still too far north. Even the southernmost location in the continental United States Ballast Key, Florida at 24.5 degrees north is just about one degree too far north. The southernmost point of the entire United States is at Ka Lae, Hawaii at 18.9 degrees north, so clearly the Sun will reach the zenith at this location and at other places on the big island just north of this site.
The good news about the summer solstice, of course, is that the amount of darkness per day increases from now until December 21, so we’ll have a bit more dark time every day. How much more dark time? Well, the velocity at which the Sun moves south in the sky varies over that time, but, on average, Houston gets about 1.6 minutes more of darkness every day between now and December 21. This is based on astronomical twilight, which is when the sky gets as dark as it’s going to get (not considering moon phase). On June 21, we got just over 6 hours of darkness and on December 21 we’ll get almost 11 hours of darkness. We’ll have two and a half hours extra of darkness in the evening and two and a half extra hours of darkness in the morning. I’m not taking into account the changes in clock time associated with daylight saving time.
If you live north of Houston, the change in dark-time between summer and winter is even higher.
The north to south (summer to winter) movement of the Sun from our viewpoint is a manifestation of the tilt of the axis of Earth. On the first day of summer, the southward velocity of the Sun is near zero. At the autumnal equinox on or near September 21, the southward velocity is a maximum, at the winter solstice the north-south velocity of the Sun is near 0 and it’s a maximum south to north velocity at the vernal equinox on or near March 21. I calculated the change in hours of darkness (based on USNO data) over the year and plotted the result. As you might expect the result looks like a familiar sine wave. There would be some variance from a sine curve because the orbit of Earth around the Sun is slightly elliptical, not circular.
The US Naval Observatory has a web site that shows hours of darkness or daylight for various cities in the U.S. and around the world. The numbers in their calculations don’t match mine because I’m using astronomical twilight time and they’re using (essentially) sunset time. (Search for “USNO hours of darkness” and you’ll find the web site.)
By Bill Pellerin
Houston Astronomical Society
If you have a world globe in your house it may include a strange figure 8 pattern, called the Analemma, with some dates on it. Without going into great detail, this shows the north/south position of the Sun in the sky for the year. Because the orbit of Earth around the Sun is elliptical, the Sun is not always on the meridian at mid-day. In fact, the Sun is only on the meridian at mid day on the two equinox days and on the two solstice days. To keep ourselves going to meetings our time clocks show the average length of the day as 24 hours. Close enough. On some days the Sun is east of the meridian at noon and on some days it’s west of the meridian at noon (always ‘standard’ time). The Analemma shows you how this goes.
So for the next 6 months, revel in the notion that you’re getting more dark sky every day. Take advantage of it.