Is it true that Spring is arriving early this year?
If the Earth's orbit were a perfect circle, and it took our granite planet 360 days to revolve around the Sun, the dates to start each season would occur the same day each year, since each month would consist of 30 days and each season would last precisely 90 days. Wouldn't it be nice if indeed the number of days it took to move once around the Sun was also the same number of degrees in a circle? But alas, things aren't always so straightforward.
Our orbit, while pretty close to being circular, is nevertheless a bit out of round, and it takes approximately 365 and 1/4 days (365.24219 to be more exact) to complete one revolution about the Sun. These two facts conspire in such a way that not only do our 12 months not have the same number of days, which is annoying enough, but also, some years are slightly longer than others. By the way, the length of time from one vernal (spring) equinox to another is referred to as the Tropical Year, and this period is the basis for the calendar we use today -- the Gregorian Calendar.
Most of us, I hope, realize that the Earth's axis is tilted or tipped about 23 1/2 degrees from the plane Earth orbits around the Sun. This tilting is responsible for the seasons and governs the amount of light we receive during the course of the year. At the time of the equinoxes, we're tilted to the side (neither facing toward or away from the Sun) so that everywhere on Earth experiences nearly 12 hours of night (hence the term equinox). Actually, we catch a break here since because of atmospheric refraction of sunlight when the Sun nears the horizon, it's up a tad more than 12 hours. Thus, if you live in near Washington D. C. (latitude of 39 degrees North), those days with almost exactly 12 hours of light and 12 hours of darkness (when the Sun is completely below the horizon) occur about March 17 and September 26.
The definition of the vernal equinox is when the ascending node of the ecliptic (the path the Sun appears to take across the sky) intersects the Celestial Equator (the position the Equator would assume if it was projected against the sky). Ok, I can tell that your eyelids are beginning to droop (mine too), but the important thing to remember here is that this marks the start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. Also, this phenomenon can't happen the same time each year since our orbit around the Sun isn't a perfect circle.
In addition, to keep our calendar in sync with the position of the Earth in relation to the Sun, a leap day is required about every four years, and thus, the commencement of the seasons shows a progressive change due to the leap year cycle. If we conveniently neglected this nagging fraction, in 400 years, the equinox would be a full season (100 days) out of kilter. Day and night would be about 12 hours long at the end of June, and so that summer vacation would seem more like spring break.
This year, the Sun will be directly overhead a point on the Equator on the 20th of March at 06:49 Universal Time (1:49 a.m. eastern standard time or EST). Note that this is the earliest date for the start of spring since 1896, when it began on March 19 at 9:24 p.m (EST). Last year, spring arrived March 20 at 8:00 p.m (EST), and in 2002, the start of spring occurred also on March 20 but at 2:16 p.m (EST). In 2005, spring arrives on March 20 at 7:33 a.m. (EST), and in 2006, it'll begin on March 20 at 1:26 p.m. (EST).
Have you noticed a particular pattern or rhythm to the above beginning dates of the vernal equinox? There's about a 6 hour difference from one year to the next when the Sun is directly overhead the Equator. Let's see 6/24 = .25. Does this number seem familiar?
However, it should be noted that the time difference for the start of spring from one leap year to the next leap year is only on the order of about an hour or less. For example, while the start of spring this year will occur on March 20 at 1:49 a.m. (EST), in 2008, spring begins on the 20th of March at 12:48 p.m. (EST), and four years later, it'll start just a little after midnight (12:14 p.m. EST).
So maybe it has become a bit more clear now that during a leap year spring occurs at an earlier time than during non leap years. This makes sense since the addition of leap day in effect means that our winter is a day longer, but to make up for this insult, spring arrives sooner than in non leap years. Of course, this means that the start of summer, fall and winter also make earlier appearances when it's a leap year. As was mentioned above, in 1896 (also a leap year), spring occurred on the 19th of March, and in 2096 (another leap year) it'll again arrive on March 19. By the way, in 1903 (a non leap year) spring was especially late, occurring on March 21.
Some of the signs that we associate with spring include the welcome trill of the spring peepers (tree frogs), crocuses, daffodils and forsythia displaying early-season color, and robins flying north. Each of these are in their own way indicators of the onset of spring, but the dates when the peepers first peep and the crocuses first emerge vary from year-to-year, and so it is with the astronomical arrival of spring, which varies between March 19 and March 21. After all, even the swallows of Capistrano don't actually return to the San Juan Mission the same day each year (March 19 -- in most years).
See also the Naval Observatory Web Page: (http://www.usno.navy.mil/faq.shtml)
and this web site about the swallows of San Juan Capistrano: (http://www.infoplease.com/spot/swallows1.html)
This week's question is provided by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! This year Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.