What's an Equinox?

In advance of the traditional American schedule, we have set our clocks ahead. But throwing a whole extra day into February will not fool nature. By celestial definition, a seasonal transition is marked neither by a number on a dial nor by height in a thermometer tube. A season, rather, follows the progress of the sun across Earth’s tilted middle.

Next Thursday, March 20, marks the astronomical beginning of spring: the vernal equinox. Although an equinox is not an especially visual event, its semi-annual arrival celebrates some of the symmetries and relationships at the heart of astronomy’s appeal.

The deep sky is where time and space mingle. We embrace this delightful mix at the start of both autumn and spring: An equinox is both a particular point in time and a particular point in space.

At 1:48 a.m. (Eastern Daylight Time), on Thursday, the rays of our sun will slam directly and perpendicularly on Earth’s equator. If you were to visit the equator and lie face-up at that time (astronomical noon) this Sunday, you would find the sun all the way at the tippy-top ("zenith") of the sky. Its beams would shine straight down on your forehead. For all other latitudes on Earth, the sun will be lower in the sky and its rays will graze rather than attack Earth’s surface. (For us here at approximately 42 degrees above the equator — as for our counterparts at 42 degrees below — we should not lie down to find the sun at noon. We should angle our heads up no higher than 90 – 42 degrees — that is, 48 degrees above the horizontal.)

The significance of the equinox is that Earth’s northern hemisphere gets no more direct (perpendicular) sunlight than does the southern hemisphere. When the sun reaches the top of our sky, it will center quite close to due south. It seems that, at least approximately, the sun should rise earlier than noon by the same amount that it sets later than noon — producing a 24-hour period equally divided between day and night. ("Equinox" etymologically refers to "equal night.")

For a variety of reasons, including the human imposition of time zones and daylight saving time, the "equal night" does not work out perfectly, but it does work out reasonably.

On Sunday, the first day of autumn, the sun will rise at 6:56 a.m. This is just four minutes before what would be the "perfect" time if we hadn’t jumped to Daylight Saving Time so early this year. It will set at 7:05 p.m. (just 5 minutes after "perfection"). Each of these events will occur singularly near due east and due west, respectively.

For sunlight to hit us in the above fashion, the sun itself must aim from a specific location. This position, currectly found in the western portion of the constellation Pisces, is a place literally called "the vernal equinox." It is the celestial globe’s equivalent of Earth’s prime meridian.

When the sun is there, hope springs here.

 Dan Yaverbaum is director of the Dixon Observatory at Berkshire School. If you have questions about matters celestial, send them to Compass@lakevillejournal.com.