Each of our seasons starts with a special sun event. Both spring and fall begin when the sun’s direct rays cross the equator. For an instant, the sun’s energy is divided equally between the northern and southern hemispheres.

On these days, the sun rises due east and sets due west. During spring, daylight increases each day and night time shrinks. Summer starts when the sun’s direct rays reach farthest north (latitude 23.5 N).

All through summer, the amount of daylight each day shrinks and night time lengthens. Then at the start of fall, the sun’s direct rays cross into the Southern hemisphere as daylight each day continues to shrink and the nights become longer.

Then at the start of winter, the sun’s direct rays shine farthest south (latitude 23.5 S). This shift in the sun’s behavior is caused by the tilt of the Earth’s axis as we travel along our solar orbit.

During winter, the amount of daylight each day slowly increases while the nights lengthen. At the start of winter, our part of the world is tipped most away from the sun while at the start of summer, our area is tipped most towards the sun.

At the start of spring and fall, we are in neutral positions, with the axis tipped side wards to the sun’s direction.

What’s not well known is that our seasons vary in length. Spring is 92.8 days long, summer at 93.6 days, fall has 89.8 days and winter is 89 days long. This variation is due to the Earth’s orbit being elliptical. In early January, we are closest to the sun, at a distance of 91.4 million miles.

The Earth’s motion about the sun is then fastest, causing winter to go by more quickly than any other season. In early July, we are farthest from the sun, at a distance of 94.5 million miles. The Earth then moves most slowly, causing summer to be the longest season.

What about our days? To explore this question, we need to realize that there are several different days: the solar day, the 24 hour day and the sidereal (or star) day.

The solar day is the day of the sundial, its length being the time between the sun’s passage due south (when it’s highest) to the next time the sun passes due south.

The solar day can be as long as 24 hours 30 seconds (in mid-December) and as short as 23 hours 59 minutes and 38 seconds (a few days before the start of fall). What causes this variation?

It is a combination of the Earth’s varying speed along its orbit, changing the sun’s apparent eastward drift and the motion of the sun along its path.

Around the start of spring, the sun moves more northward than usual while around the start of fall, the sun moves more southward than usual. Northward motion will increase daylight while southward motion will decrease daylight.

This is similar to the Harvest Moon effect when the full moon around the start of fall lingers longer in the evening sky. These two effects are responsible for the change in the solar day.

The 24 hour day is based on an imaginary sun moving the same angle eastward each day of the year along the celestial equator. (The celestial equator is the extension of the Earth’s equator onto the sky.) This uniform motion causes a 24 hour day, the basis for our clock time.

The sidereal (or star) day is the true rotation period of the Earth, which averages 23 hours and 56 minutes. How can this be true? Just set up a telescope in the evening and note the time when a bright star passes across the middle of the field.

Suppose this crossover occurs at 10 p.m. Then the next evening, you will see the same bright star cross the middle of the field at 9:56 p.m. This four minute difference is caused by the solar day or 24-hour day being based on the appearance of the sun.

Because of our motion about the sun, the sun seems to drift eastward about one degree a day. So it takes the Earth an extra four minutes to restore the sun to its same position as the previous day.

This four minute difference accumulates to one entire rotation per year. So each year, we have 366.25 Earth rotations, not 365.25 as most people believe.

Today is Easter Sunday. This special occasion was triggered by the full moon last Friday.

Easter falls on the Sunday following the first full moon on or after March 21. This rule was set up by the Council of Nicea in 325 CE. Easter can occur as early as March 22 or as late as April 25.

Reference: Fundamentals of Astronomy by C. Barbeiri, 2007, Taylor and Francis.

EVENING SKY THIS WEEK: Early this week, the moon quickly leaves the evening sky, rising more than an hour later each night. Tomorrow, the moon will be rising about 11:45 p.m.; the next moonrise will be 12:45 a.m. on Wednesday.

This coming weekend, the brilliant planet Venus will appear to the right of the bright star Aldebaran (the eye of Taurus). The planet Jupiter this week sets about 9:45 p.m. and will be lost in the sun’s glare by the end of April.

Our Animal-Sky program will not be held this afternoon to allow observance of Easter.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.