20 March 2019

When is an Equinox not an "equi"-"nox"?

Image Public Domain
Most folks know that the word equinox comes from Latin, meaning "equal (equi) night (nox)." We talk about it as one of the two days of the year when there is an equal amount of daylight and darkness over the whole Earth. We're taught that the Sun spends exactly twelve hours above the horizon, from its time of rising in the east, to its setting in the west.
However, that's not really true. In my location in middle New England, the day and night were equal about four days ago. In more southerly regions, it might have been between 5-7 days ago.
How can this be? The discrepancy is due to the fact that we measure the Sun in a couple of different ways. The Sun takes up about one degree on the sphere of the sky, and we measure its location at the center of the Sun's disk. But the upper edge of the Sun will come over the horizon about two minutes before the center of the disk aligns with it. And because light curves around the horizon, it will appear that the Sun is starting to rise a few minutes before that. And we don't consider the Sun as having fully set until the entire disk drops below the western horizon, again a few minutes after the center of it aligns with the horizon line.
Of course, some of these factors will change depending on what your local horizon looks like. If you live in a valley, or have thick bushes or trees on the eastern or western horizon, then the rising will be delayed somewhat and/or the setting will happen a bit earlier.
But the good news for all you Sun worshipers out there is that the days are already more than half light than darkness. Here in the foothills of the Berkshires, there is still some snow on the ground, and we are expecting one or two more snowfalls of an inch or two in the next couple of weeks. But the daffodils and crocuses are insisting that Spring has indeed arrived!