Why is there "equal night"?

Question

The fall equinox occurs on or about Sep 22. Equinox meaning, "equal night", and this day is defined to have 12 hrs of day light and 12 hrs of darkness. However, consulting the sunrise and sunset data for this day, sunrise is 640AM and the sunset is at 648pm! Why aren't the sunrise and sunset times identical, or better yet, why is daylight longer than 12 hrs???

Answer 1

Great question, Mariana!!!

You are EXACTLY right when you say that the fall equinox occurs "on or about" Sept 22.

The equinox occurs at EXACTLY some particular TIME on that day. You could try picking out another time zone to check on that day, and I'll bet some would have a day closer to 12 hours, and some a day even longer than 12 h 8 min. (and some a day LESS than 12 hours)

(kind of like an eclipse might happen on a certain day, but WHEN on that day is important)

Sometimes they will report that fall begins at such and such a time on Sept 22 this year.

For a good reference see below. Good luck!

Try it out with some time zones about 3-6 hours distant in each direction.

Hope that helps! :-)

(also the exact time of sunrise and sunset have to do with how far east and west you are in the timezone. eastern cities in their timezone will have both a sunrise and a sunset earlier in the day, and western cities will have both later in the day. shoot for one right in the middle of a timezone for the best hit)

Answer 2

Basically, the MAIN reason is because the atmosphere refracts (or bends) the light from the sun, making it appear to be above the horizon and not yet set, when in reality it already has. It similarly appears to rise before it actually has too for the same reason adding "daylight time". In other words, if the earth had no atmosphere, it would be closer to 12 hours. Another factor, is that the equinox occurs at a specific instant in time, not over an entire day. So, at some places on earth, the instant of equinox happens at midday, some just after midnight on the "date of equinox" and others just before midnight on the "date of equinox". That means that in some places the sun is north of the equator for MOST of the "date of equinox", in others it is south of it MOST of the "date of equinox" which affects the total time of daylight for those locales. (Remember because of 24 time zones around the earth, equinox doesn't happen at the same CLOCK time for each of them!)