Which of the following are connected in some way: "tax day," the Ides of March, the Vernal Equinox, opening day in baseball, Passover, and Easter?

All of the above occur during about a 5-week period but only the Vernal Equinox, Passover and Easter are actually related to each other. Some have been advancing over the years (tax day), others have been receding (opening day in baseball) and still others vary from one year to the next (Vernal Equinox, Passover, Easter). Of the above, just the Ides of March is set to a given calendar day.

Most of us look forward to the arrival of spring, despite the coincident appearance of pollen and resulting allergies and the fact that taxes are due. Spring fever gives us a taste of what we hope will soon follow -- those lazy, hazy, crazy days of summer. To our ancient ancestors, spring was more than an end to winter -- it was synonymous with renewal and life itself. In early Roman times (2,500 years ago), March was the 1st month of the year and not relegated to 3rd in the line up -- the Equinox and arrival of spring was an esteemed occurrence, worthy of the #1 slot.

Today, some people mark the start of spring with the chorus of the first spring peepers, the blooming of the first crocuses or maybe even the first mosquito bite. Depending on the severity of the winter, these arrival dates will change from year to year. But you would think that the official astronomical arrival of spring (Vernal Equinox) would be firmly set. After all, the Sun and stars seem to change very little. Interestingly, though, the vernal equinox doesn't occur at the same time or even sometimes the same day as the year before.

On about March 20, the Sun is directly above the Equator. But why doesn't the vernal equinox happen at the same exact time each year? If the Earth were a tad closer to the Sun, so that our period of revolution was exactly 365 days, then things would be a little different. However, remember that it takes approximately 365.25 days to return to the same point in our orbit, and that the Earth is both revolving around the Sun and spinning on its axis (rotating) at the same time. So from one year to the next, the time and place that the Sun is precisely above the Equator will vary. 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. So, while spring began this year precisely at 7:33 a.m. on March 20th (eastern standard time), it occurred on March 21 in 1903 and will occur on March 19 in 2096; two full days earlier (regardless of what Punxsutawney Phil prognosticates).

Because the Earth's axis is tilted 23 1/2 degrees, those of us who live in places other than the tropics experience changes in weather and the amount of daylight from one season to the next. Shortly after the Sun moves northward of the Equator, our days begin to noticeably lengthen. However, the reason that Daylight Savings Time (DST) has been adopted in the U.S. (Hawaii and Arizona aren't on DST) has to do more with social affairs rather than astronomy. The key thing about DST is that we get to use the additional light available in spring and summer during the hours we're awake. On the first Sunday in April the clocks are moved forward one hour. The result being that at noon the Sun is no longer at its apex, rather it reaches the highest point in the sky at 1:00 p.m. Of course, we don't actually get any more light by adjusting the clocks, it's just redistributed.

March 15 is the Ides of March, which while being near the date of the Equinox, isn't really related to the onset of spring. The Romans didn't count the days of the month from 1 through 30. Instead, they used a system whereby certain days were assigned names, and these names were then used to refer to the other days of the month. The Kalends fell on the 1st of each month, the Nones on the 5th or 7th of each month, and the Ides on the 13th or 15th. For example, the 16th of March was referred to 17 days before the Kalends. The Romans evidently didn't like the idea of counting backwards, so the 16th of the month was never called the day after the Ides. Our familiarity with the Ides of March is due to the soothsayer's warning to Julius Caesar. "Beware the Ides of March. Ok, it was a bad day for old Caesar, but for the rest of us, it's a day that should be no more guarded than the Kalends, the Nones or those days in between. Nonetheless, the "Ides of March" still connotes a feeling of dread .

Baseball's opening day has long been a sign that spring is in the air. Once the "American Past time," baseball has lost some of its prestige in recent years because of escalating salaries, 9 inning games that often take 3 1/2 hours to complete, and a 162 game, 7 month season. Initially, the season started when it was typically warm enough for fans in places like Boston, New York and Chicago to fill the stadiums -- mid April. Now, the first pitch is thrown as early as March 30th in some years. This slide toward the Vernal Equinox has nothing to do with climate warming -- a longer season of course means more revenue for owners and more money for the players.

This year, after an absence of 34 years, the city of Washington finally has a Major League team -- the Washington Nationals. The Nats moved south to D.C. from Montreal not because of global cooling (the Montreal Expos played indoors), but because for some reason the fans in Montreal would no longer support a team that had a loosing record year after year after year. Last night in the Nats first home game of the season, they defeated the Arizona Diamondbacks 5-3 before 46,000 fans to take over first place in their division. Go Nats!. Once the weather heats up in D.C., the "Gnats" will indeed be a fitting nickname.

Easter and Passover are linked to the Moon's appearance after the Equinox. Easter almost always occurs on the first Sunday following the first full Moon after the Vernal Equinox. While Easter occurred on March 27 this year, Passover isn't until April 24 -- about as late as it can occur. The rule for establishing the date for Passover, which was originally intended to track the vernal equinox, is off several days now. In ancient times, this wasn't a problem because Passover was set by actual observations of the Moon and of the vernal equinox.

The Hebrew calendar is based on the Moon's rhythms and is somewhat similar to the Islamic calendar, but it's more complicated. This calendar is a combined solar/lunar calendar and uses a formula to adjust time in such a way that the holy days or festivals, such as Hanukkah, occur during the same season from one year to the next. An ordinary year has either 353, 354 or 356 days, but in the Hebrew calendar, leap years having either 383, 384 or 385 days are employed to keep the calendar in accord with the Earth's revolution about the Sun. Leap years have 13 months rather than 12, though, they don't occur at regular 4 year intervals as they do in the Gregorian calendar. Because every 19 years corresponds to exactly 235 lunar cycles, the Hebrew calendar consists of 12 years with 12 months and seven years with 13 months. Leap years occur in the 3rd, 6th, 8th, 11th, 14th, and 17th years of the 19 year cycle. Passover occurs the same time each year according to the Hebrew calendar, but because the Gregorian calendar is not in sync with the Hebrew calendar, the dates for Passover vary from one year to next on the Gregorian calendar.

Tax Day (April 15) has nothing to do with spring, except that it occurs in the middle of a spring month, seemingly placed here for the sole purpose of dampening our spring fever. The idea of a tax on income was first thought up to help pay for expenses related to the Civil War, but it took until 1909 before the 16th amendment was passed by Congress. It was ratified on February 3, 1913 -- in the dead of winter. Initially, the due date for the previous year's income tax was March 1, but in 1918 it was pushed forward to March 15 (Ides of March). Alas, it was moved to April 15 over 50 years ago -- in 1954. By the way, the Ides of April are on the 13th not the 15th. For some folks, neither the Ides of March nor triskaidekaphobia (fear of Friday the 13th) evoke as much trepidation and foreboding as April 15 (taxtodeathaphobia).

For more about the Ides of March see -- http://www.travel-italy.com/ct/episodes/ides.html

For more about calculating dates for Easter and Passover see -- http://quasar.as.utexas.edu/BillInfo/ReligiousCalendars.html

This week's question from is provided by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for EIGHT 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.