Equinox Day
Published on Mar 20, 2006 at 12:44 pm.
3 Comments.
Filed under Earth.
Today is the vernal equinox. For those of us in North America, it marks the first calendar day for spring. Personally, I always thought that was dumb. Spring weather already has arrived by March 21. Winter weather arrives well before December 21. Summer is here before June 21, and it is generally over by the time the Autumnal equinox occurs. Really, the “seasons” should start and stop on the first day of the month that the equinoxes or solstices occur. But, as usual, no one asks me for a rational way of doing things. I really need to get a move on with the taking over the world so that I can put everything into order. 
The term “equinox” comes from the Latin for “equal nights.” This makes sense, because the equinox is defined as the point in time when the Sun appears to be on the Celestial Equator. Such a position is above the horizon for 12 hours, and below the horizon for 12 hours. So, there are 12 hours of daylight and 12 hours of night today, right? Well, look at the sunrise and sunset times in your local newspaper. You will find that, in fact, sunset occurs slightly later than 12 hours after sunrise. Huh? Maybe yesterday or tomorrow will be the day of 12 hours of daylight and night. Nope. That was several days ago for those of us in the Northern Hemisphere, and it will be in several days for those of you in the Southern Hemisphere. What gives? Well, it turns out that there are two factors that mess up the “equal days and equal nights” thing for the equinox. First of all, the Sun is not a point source. It subtends about half a degree of arc in the sky. Sunrise occurs when the first edge of the Sun rises, and sunset times occur when the last of the Sun has disappeared. The equinox is then the center of the Sun is on the celestial equator. The edge of the Sun first appears about a minute before the center of the Sun, and the last edge disappears about a minute after the center. So, the day of equal night and day will be a day or two on the winter side of the equinox. Secondly, atmospheric refraction needs to be accounted for. The atmosphere bends light, and this has the effect of “raising” objects seen near the horizon. In other words, they appear slightly higher than they really are. So, when, from you location, the center of the Sun is on the horizon, you see the Sun a little higher than the horizon. Actually, this effect raises the Sun at least its own width, so really the Sun would geometrically be below your horizon by the time that you see the edge of it touch the horizon. This means that the Sun will appear to be above the horizon for several minutes extra due to the effects of the atmosphere. Taking that into account pushes the day of equal night and day back another day or two towards the winter side of things. So, equal days and nights really occur a little towards the winter. That has the effect of lengthing the Spring and Summer and shortening the Winter and Fall. That statement holds for both the Northern and Southern hemispheres.
Often introductory books will say that the North and South poles have six months of daylight and six months of night. Again not true for the very same reasons. Really, there are about seven months of daylight, a couple months of twilight, and only a bit over three months of night.
The equinoxes and solstices happen because the Earth is tilted about 23.5 degrees in its axis of rotation relative to the orbit. The solstices are when the Earth’s poles are tilted either towards or away from the Sun. The equinoxes are when the Sun is over the Earth’s equator. OK, so it takes a year for the Earth to go around the Sun. That means that from equinox to equinox or from solstice to solstice is half a year, right? If you count the number of days from the Winter Solstice to the Summer Solstice, and then count the days from the Summer Solstice to the Winter Solstice, you will find that the two numbers agree to within about a day. This is to be expected. You don’t expect exactly the same number of days, after all, since the year does not have an exact even number of days in it. So, you are convinced that the statement that between solstices is half a year is valid. Next, you count the days from the March equinox until the September equinox. Then, you count the days from the September equinox until the March equinox. Now, you find a surprise. It takes about a week longer to go from the March equinox to the September equinox than it does from the September equinox to the March equinox. What gives? You count again, and you get the same results. Hmm. Well, this is because Earth’s orbit is not a perfect circle. Our orbit is actually slightly elliptical. Earth is closest to the Sun in early January, and farthest from the Sun in early July. As planets orbit the Sun in their elliptical orbits, they speed up and slow down depending upon their distance from the Sun. Planets move slower when they are farther from the Sun than they do when they are nearer. So, since Earth is farther from the Sun in January, it is moving slower in November, December, January, and March than it is in May, June, July, and September. Thus, the summer in the Northern Hemisphere is longer than the winter, and the winter is longer in the Southern Hemisphere than the summer. That is all due to the eccentricity (not a circle) of the Earth’s orbit. Since the times of closest and farthest from the Sun are fairly near the Solstices, then the effect is much less apparent when counting from solstice to solstice.
So, that’s your equinox lesson for the day!
-Astroprof
Astroprof’s Page » Vernal Equinox on March 20, 2007 at 11:05 am: 1
[…] It happens in a few hours. At 00:07 UT, March 21, the Sun will be directly over the Earth’s equator. That time works out to be 7:07pm (CDT) on March 20 here in Texas. We call this the Vernal Equinox. The word “equinox” suggests equal times of day and night, and that is what many introductory texts say about this day. However, as I posted a year ago, that is not correct. The day of equal day and night was a few days ago (how many days ago depends upon your latitude).  Click on this link to look at my older posting to read more about the equinox and seasons. I’ll only summarize the matter here. […]
Chris D on November 8, 2007 at 3:00 pm: 2
My questions is why does the solstice mark the beginning of a season and not the middle of it? If the winter solstice is 12/21, whey do the coldest months occur after this date as the days are no getting longer and the angle of the earth to the sun is decreasing?
Astroprof on November 8, 2007 at 4:33 pm: 3
Good question. I wrote about that here: http://astroprofspage.com/archives/58