Two Noons?

A colleague recently surprised me. She posted an item about the recent equinox in a science blog and mentioned solar noon. There’s another kind of noon? Two noons? I always thought of noon as 12:00 o’clock as shown by my watch, though I wondered why a nearby sundial seemed so far off from my watch.

There is, I learned, another type of noon called solar noon. Solar noon is the moment when the sun passes directly overhead and reaches its highest point in the sky for the day. The sun is directly crossing the meridian where you are located.

Meridians are another name for the lines of longitude which run between the North Pole and the South Pole. One of these lines or meridians runs through every location on earth. You can pinpoint any spot on the surface of the earth using the correct lines of latitude and longitude, the horizontal lines, like the equator. Our GPS devices use latitude and longitude to navigate down to the street level.

In most cases, the sun does not pass over your local meridian at exactly 12:00 o’clock by your watch. Usually, solar time and clock time are different, but not by much. Solar time depends on the sun’s position relative to the earth, both of which are in motion.  Our watches and smartphones measure clock time in fixed time intervals of seconds, minutes, and hours. The official atomic clock in Boulder, Colorado, sets the standard for all US clock time.

Solar noon is different for every meridian. For example, on Friday the 30th of September in San Francisco, California, solar noon will be at 12:59 p.m., one minute before 1:00 o’clock p.m. However, solar noon in Sacramento, CA on that Friday will be at 12:55 p.m., 5 minutes before 1:00 o’clock p.m. Since Sacramento is north and east of San Francisco, it is on a different meridian and has a different solar noon. But both Sacramento in San Francisco will have the same clock time or civil time because they are in the same time zone.

The time of day before the sun crosses the line of longitude was called by the Romans, ante meridian, or before the meridian. Similarly, the time past solar noon was called post meridian or after the meridian. We still use these terms in abbreviated form today with clock time when we name a time as 10 a.m. or 4 p.m., ante meridian or post meridian.

Clock time and solar time may also differ because of Daylight Savings Time (DST) and clock time now appears almost an hour ahead of solar time. In November, when DST ends for most of the US, my watch and the sundial in the park will be in closer agreement. But not exactly. That will only happen once in a blue moon, figuratively speaking.