Expert: Philip Stahl - 3/9/2009

Question
In my 8th grade Earth Science class, we are looking at sunrise and sunset times.  On March 17, the sun will rise at 7:28 am and set at 7:28 pm - 12 hours of daylight and 12 hours of dark?  Shouldn't this happpen on the equinox, March 21st?  On March 21st, the sun will rise at 7:22 amd and set at 7:32 pm, a daylight time of 12 hours 12 minutes.  This confuses the meaning of equinox as "equal night"

Answer
Hello,

You ask a good question. It is true that the  also implying "equal day") but as they say, the devil is in the details.

In astronomical terms, the equinox date (and time!) is specifically set at a precise astrometric instant: when the center of the Sun crosses the celestial equator going northwards. This means the center of Sun must cross a declination of D = 0 degrees, for that to occur. It does on March 21st (usually).

However, one will still not see an exact equality of daylight (say from 6 a.m. to 6 p.m. standard time) as theoretically predicted.

There will be some inevitable lengthening as you have noted. This arises first from refraction effects, since the Sun when setting will be a slight bit longer in doing so because of this. By the same token it will be a slight bit delayed on rising for the same reason. (Refraction describes a "bending" of light rays entering the atmosphere owing to the fact the latter has a greater density than the vacuum of space through which the sunlight has transited)

Another fact to note (for your students' benefit) is to make sure they know times of sunrise and sunset are calculated from the solar limb position, not its center. Since the Sun is 1920 arcsec across, this means a deviation in time corresponding to 1920"/ 2 = 1810" of arc.

Since 4 mins. of time corresponds to 1 degree of angular separation (roughly) then this will tack on an additional time of more than 2 minutes to each of the sunrise-sunset computations. That is, adding more than 4 minutes of total time differential to the theoretical day length (2+ mins. at sunrise - since the rising solar limb reaches the horizon that much time before its center; 2+ minutes at sunset- since the setting solar limb reaches the horizon that much time *after* the solar center))

I believe the point to convey to your students is that there are many things (especially in astronomy) that aren't quite so simple or straightforward as they first appear!