Vernal Equinox
Published on Mar 20, 2007 at 11:05 am.
9 Comments.
Filed under Earth, astronomy.
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.
The equinox is defined as the moment that the Sun appears directly overhead at Earth’s equator. Because Earth is tilted on its axis, the Earth’s north pole is tilted towards the Sun from about the third week of March until about the third week of September. From about the third week of September until the third week of March, the Earth’s south pole is tilted towards the Sun. The Sun is overhead at the equator only two days per year, and these are the equinoxes. The Vernal Equinox is the one that occurs in the Spring, and the Atumnal Equinox is the one that occurs in the fall. Traditionally, these dates are supposed to be March 21 and September 21. However, since the Earth does not orbit the Sun in an exact integer number of days, those dates are only approximate (and the Autumnal Equinox more often falls on Sepetember 22 than on September 21, and sometimes as late as September 23). This year, the Vernal Equinox occurs 00:07 UT on March 21, so about half of the world will see the equinox on March 21, but the other half will see it on March 20. The Autumnal Equinox this year will occur at 09:51 UT on September 23 (that will be 4:51am CDT here in Texas).
But, the term Vernal Equinox has more than one meaning to astronomers. As I have indicated above, it is the moment in time that the Sun appears to be over the equator, as the Earth moves around the Sun, passing from when the south pole is tipped more toward the Sun to when the north pole is tipped more toward the Sun. But, there is another meaning. The term Vernal Equinox not only applies to a moment in time, but also to the apparent spot on the celestial sphere that the center of the Sun appears to be at the moment that it is directly over Earth’s equator. This spot on the celestial sphere where the Sun appears to be at that time is called the Vernal Equinox, so when you see the term written down, you need to look at it in context to see if it refers to a moment in time or a spot on the celestial sphere. The postion of the Vernal Equinox on the celestial sphere is the starting point (or prime celestial meridian) for the celestial coordinate of right ascension. Just a few weeks ago I posted a bit about celestial coordinates. So, the celestial prime meridian is called the vernal equinoctial collure.
I guess that it seemed to make sense to people a long time ago to put the celestial prime meridian at the Vernal Equinox. The problem is that its position moves in the sky! As Earth orbits the Sun, the direction of its tilt changes, in a physical process called precession. The reason for this is much like the reason that a top wobbles if it is leaning over while spinning instead of falling down. But, if you change the direction of the tilt, you will also change when the Sun is overhead at the Equator. We fix the calendar to the seasons, so that is adjusted by adding days to the year as needed to keep everything in sync. But, the position in the sky changes, and thus so does the vernal equinoctial collure. Thus, all the celestial coordinates of fixed objects will change! Really, I don’t think that this is a good way to do celestial coordinates!
There is yet another effect that changes when and where the equinoxes occur. As a planet orbits the Sun, the orbit does not repeat exactly. Instead, the whole orbit slowly slings around, and the point where the perihelion (closest point to the Sun) and aphelion (farthest point from the Sun) slowly shift. Elliptical orbits speed up and slow down. Right now, the Earth is moving slowest during July and fastest in January. That makes the summers in the northern hemisphere longer than the winters by a few days (and the winters longer than the summers by a few days in the southern hemisphere). If you count days, there are more days from the Vernal Equinox until the Autumnal Equinox than there are from the Autumnal Equinox until the Vernal Equinox. The calendar adjusts for that by varying lengths of days of the months, but as time passes this shift in the position of perihelion and aphelion will cause the length of time between the equinoxes to shift. That means that the dates of the equinoxes will continue to shift so that the Atumnal Equinox will come earlier and the Vernal Equinox later. But, that will be a very long time in the future, and not something for you to worry about.
Oh, and the whole egg on its end thing at the equinox … that doesn’t work. I have no idea how that idea got started.
-Astroprof
Diagram courtesy of NASA
A Ler…-- Rastos de Luz on March 20, 2007 at 5:39 pm: 1
[…] “Vernal Equinox“, no Astroprof’s Page. Também a ler neste blog, “Titanic Lakes“. […]
Kate on March 21, 2007 at 10:56 am: 2
Could it be that the ancient observatories were built in order to track the precession
and thus make future civilizations aware of
earth changes that are forseeable natural occurances?
Astroprof on March 21, 2007 at 12:23 pm: 3
I don’t know if they built observatories just to track precession, though certain ancient astronomers (like the Egyptians) clearly knew about it.
Astrolink on March 21, 2007 at 1:42 pm: 4
Hello
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Cheers
Lab Lemming on March 30, 2007 at 6:24 pm: 5
Isn’t the Juno mission supposed to measure all sorts of fundamental parameters of Jupiter’s rotation, magnetic field, etc?
As for saturn, can you estimate the rotation period from the planetary oblateness?
Astroprof on March 30, 2007 at 7:03 pm: 6
Yes, Juno (if it actually flies) is supposed to provide detailed measurements of Jupiter. But, you can not compute rotation from oblateness unless you know details of the planet’s interior, which we don’t for Saturn.
Lab Lemming on March 30, 2007 at 11:20 pm: 7
Can you get angular momentum from cassinni’s orbital procession? What else do you need?
p.s. Sorry, I seem to have commented in the wrong thread.
Astroprof on March 30, 2007 at 11:27 pm: 8
In theory, yes. In practice, though, Cassini was not built with the precission to make those measurements (Gravity Probe B was special made for just those sort of measurments). Though, I am sure that they’ll be monitoring Cassini’s orbital motion to see if there is anything at all that they can glean to shed light on what is going on inside Saturn. But, the guys at JPL put it best themselves. They said that if the magnetic field doesn’t rotate with Saturn, then there is no way to really tell yet what Saturn’s internal rotation is.
julia on August 5, 2007 at 10:13 pm: 9
it is so interesting to recognise the earths orbit at the vernal equinox process..but am wondering what is the length of days at the Solomon Islands and those at Vostok station of antartic…