Jimmy Westlake: The longest day of the year
June 21, 2011
Steamboat Springs — The summer solstice, marking the official end of spring and the beginning of summer in the northern hemisphere, is at 11:16 a.m. today. The summer solstice is sometimes referred to as the "longest day of the year," but aren't all of our days exactly 24 hours long?
Yes, they are, but ever since the winter solstice last December, the time interval between our sunrise and sunset has been increasing, giving us more minutes of daylight and fewer minutes of darkness each day.
So the phrase "the longest day of the year" refers to the number of daylight hours in a 24-hour period, not the actual length of the day.
Consider that on the winter solstice on Dec. 21, sunrise in Steamboat was at 8:26 a.m. and sunset was at 5:45 p.m., for a total of 9 hours and 19 minutes of daylight and 14 hours and 41 minutes of darkness. Today, the sun rose at 5:38 a.m. and will set at 8:40 p.m., giving us 15 hours and 2 minutes of daylight and only 8 hours and 58 minutes of darkness. So, we are enjoying an extra 4 hours and 43 minutes of daylight this week compared with mid-December.
That's the good news. The bad news is that, after the summer solstice, the trend reverses, and we start losing those precious minutes of daylight again as we head toward the winter solstice.
As our daylight hours increase between solstices, the noontime sun rises higher and higher in our sky, causing noontime shadows to grow shorter and shorter each day. The noontime sun in Steamboat will be 72.5 degrees high today. That's as high as it can ever get for us, so we have short, stubby noontime shadows, but they don't disappear at noon.
In about the year 200 B.C., the Greek scholar Eratosthenes heard a rumor from some travelers that at high noon on the date of the summer solstice in the town of Syene, Egypt, the shadows of vertical objects briefly disappeared. This indicated to Eratosthenes that the sun must be shining down from the very zenith of the sky in Syene. He thought this very curious, since the noontime sun on the date of the solstice from his hometown of Alexandria, Egypt, was 7 degrees off of the zenith, creating very short shadows.
Most people might have just dismissed the rumor, but Eratosthenes did not. He realized that if the Earth were flat, noontime sun angles would be the same everywhere, but if the Earth was shaped like a sphere, different locations would experience different sun angles at noon. He reasoned that the distance between Syene and Alexandria must represent 7 degrees of the Earth's total circumference, so a quick calculation produced the distance around the spherical Earth. Eratosthenes' measurement was only 2 percent off from the accepted circumference of 24,901 miles. Not bad for 200 years B.C.
Happy summer solstice!
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all across the world. Visit Westlake's website at http://www.jwestlake.com.