Home > Concepts, Ephemeris Program, Equinox, Observing > 03/17/2022 – Ephemeris – We have 12 hours of daylight and night today, three days early

03/17/2022 – Ephemeris – We have 12 hours of daylight and night today, three days early

March 17, 2022

This is Ephemeris for St. Patrick’s Day, Thursday, March 17th. Today the Sun will be up for 12 hours even, setting at 7:51, and it will rise tomorrow at 7:49. The Moon, 1 day before full, will set at 8:20 tomorrow morning.

Why is this the day of equal day and night, when the vernal equinox, which means “equal night” is still three days away? The difference is due to our atmosphere and our definition of sunrise and sunset. Our atmosphere makes objects near the horizon appear higher than they actually are, which hastens sunrise and retards sunset. Also, the instant of sunrise and sunset is when the top of the sun appears to touch the horizon, rather than when the Sun bisects the horizon. Plus, it’s moving about a degree a day (twice its diameter) against the stars. So by the time of the equinox, on Sunday the 20th, the time between sunrise and sunset will have progressed to 12 hours and 9 minutes. But it was close enough for the ancients who coined the term.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). They may be different for your location.

Addendum

This works for the Interlochen area. It may be a different by a day for other locations, but for the northern latitudes, it will be before the true equinox day, March 20th here.

Atmospheric Refraction

How the atmosphere bends the light of the Sun or Moon rising or setting to appear higher than it actually is. S is the actual position of the Sun, S’ is the apparent position of the Sun. The blue line is the observer O’s horizon. The gray line is the actual, though much exaggerated, light path bent or refracted by the Earth’s atmosphere. The black line is the apparent sight line to the Sun. Credit Francisco Javier Blanco González, 2017

%d bloggers like this: