Wednesday's equinox is not quite equal
Posted September 22, 2015
A number of astronomically significant events occur this week. There will be a total lunar eclipse Sunday evening which coincides with a supermoon (more on that later this week). The September equinox occurs Wednesday. Our not-really-equal-but-close-to-equal-day-night day occurs Saturday. I admit I made that last one up but read on.
Before 4:20 a.m. on Wednesday, the Earth’s axis has been tilting us here in the northern hemisphere a little more toward the sun resulting in longer (summer) days. During that moment, Earth’s axis neither tilts toward or away from the sun. This happens as the sun reaches a point directly above the equator. This year, that will be a spot over the Indian Ocean about 1,200 miles east of Mogadishu, Somalia. This will next happen on March 20 at the next equinox. Once that moment in time has passed, the axis tilts us increasingly away, resulting in increasingly fewer hours of (autumn) daylight.
The equinox is also marked at archeoastronomy sites worldwide.
The Chichen Itza temple in southeast Mexico is aligned to cast a snaking shadow along the stair only during the September and March equinoxes. Mayan astronomers also tracked the movements of Venus via this structure. Angkor Wat temple in Cambodia is known for its beautiful sunrises year-round, but during the September and March equinoxes crowds gather to see the sun rise directly behind the tallest tower. In Ireland, the rising equinox sun is aligned to the inner chamber of the Neolithic mount Cairn T illuminating the back wall. Many more examples exist where ancient astronomers used architecture to mark the transition of the seasons.
While “equinox” translates to “equal night” in Latin, the day and night will not be equal in length as the name suggests. This is because of the definition of sunrise and sunset, along with a bit of geometry.
Sunrise and sunset are defined as the point where the top of the sun is first and last visible respectively. That few extra minutes needed for the full disk of the sun to appear and disappear adds to the duration of daylight, about 7 minutes here in central North Carolina. Our close-to-equal-day-night day comes Saturday, Sept. 26, when the time that passes between sunrise and sunset is just about 30 seconds longer than 12 hours.
The WRAL.com WeatherBlog will have a complete guide on Thursday on how you can experience Sunday’s total lunar eclipse including skywatching events around the area where experts will be on hand with telescopes to share views of the eclipsed moon.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.