Leap Year, the Solstice, and Seasons

Young Astronomers Blog, Volume 28, Number 14.

As we go through the year, we notice that our weather changes. For those of us who live in the United States it can be quite warm (okay, hot) during the summer months. In winter, it is cold and often accompanied by snow.

As we look up at the night sky, we notice that the constellations change over the course of a year. In the winter, Orion appears to the south, followed by Leo in the spring. Then Scorpius and Sagittarius are low on the horizon during summer. In the fall, Pegasus flies high above us.

As we follow the stars, we think of them as fixed to something called the celestial sphere. Their positions are identified with coordinates called right ascension and declination, which are similar to longitude and latitude. Over the course of a year, the Sun appears to move around the celestial sphere following a path called the ecliptic. The celestial equator is the midpoint of the sphere dividing it into two the way the equator divides the Earth into two hemispheres.

We also notice that the Sun arcs across the sky each day, rising in the east and setting in the west. Of course, the Sun isn’t actually moving, we are. The Earth rotates once a day making the Sun and the stars appear to move across the sky. It takes the Earth 23 hours, 56 minutes, and 4 seconds, to rotate relative to the stars (a sidereal day).

However, it takes slightly longer, 24 hours, to rotate one time with respect to the Sun (a solar day). This is because the Earth is also revolving around the Sun and moves a bit each day. We call this an orbit. In fact, it takes the Earth 365 ¼ days to orbit the Sun one time. This is the definition of our year. That extra ¼ of a day is why we have leap year and add an extra day to February (almost) every four years.

Did you know that we do not have a leap year for years that represent the turn of a century (those divisible by 100) unless the year is also divisible by 400? For example, the year 2000 was a leap year, and 1900, 1800, and 1700 were not. This is because the Earth’s orbit is not exactly 365 ¼ days.

• The sidereal year, the time for the Earth to orbit one time relative to the stars, is 365.256 days.
• The tropical (solar) year, the time for the Earth to go through a full set of seasons, is 365.242 days.

Our calendar, used to track the seasons, is based on the tropical year. Adding an extra day every four years gets us a bit ahead, so every so often we need to skip a leap year.

This still doesn’t explain the seasons and the variation in weather we experience from season to season. We might think that it is due to the distance of the Earth from the Sun. If you have ever gone camping, you know that the closer you are to the campfire the hotter it is. When you walk away, particularly on a cold night, the heat disappears quickly.

The Earth has a very circular orbit as it moves around the Sun. It averages 93 million miles from the Sun. However, its orbit is slightly elliptical. It is 91.4 million miles from the Sun in January and 94.5 million miles from the Sun in July. So, we are actually closer to the Sun during our winter than we are during our summer. Therefore, something else must be going on.

We notice that the Sun arcs lower in the sky during the winter and higher in the sky during the summer. This is because the Earth tilts toward the Sun in summer and away from the Sun in winter. It is this tilt that gives us seasons. Yes, we are all tilted over on our side by around 23 ½ degrees. As the Earth orbits the Sun, the Sun shines more on a different part of the Earth at different times of the year.

• In the spring and the fall, the Sun shines more directly on the equator. We call this the vernal equinox and the autumnal equinox when we have an equal amount of day and night. Officially the equinox is the time when the Sun’s path on the ecliptic crosses the celestial equator.
• In the summer, the Sun shines more to the north. We call this the summer solstice and we experience long days and short nights. In the winter, the Sun shines more to the south and we have short days and long nights. This is the winter solstice. Officially the solstice is when the Sun reaches its northernmost or southernmost position on the ecliptic relative to the celestial equator.

It is interesting to note that we mark the beginning, not the middle, of each season with these dates. Spring and fall begin on the date of the equinox. Summer and winter begin on the date of the solstice. This is because it takes a several weeks for the oceans and the land, to a lesser extent, to warm up or cool down. Therefore, our seasons are in line with the expected high or low temperatures. This is called the “seasonal lag” or “lag of the seasons.”

Meteorologist, those folks that study the weather, track the seasons using full months. For example, spring is in March, April, and May. This is called Meteorological Spring as opposed to Astronomical Spring, which begins with the vernal equinox.

It is not the same for everyone across the globe. Seasons in the southern hemisphere are opposite of the seasons in the northern hemisphere. For example, when the Sun shines more to the north, we have summer while they have winter. When the Sun shines more to the south, they have summer and we have winter.

There are a few more subtle complications to all this.

• The Earth is tilted by 23 ½ degrees, but the tilt does swing a bit. Over a 41,000-year period, it will vary between 22.1^o and 24.5^o.
• The Earth also wobbles. Today the north celestial pole points toward a spot near the star Polaris, which we know as the North Star. This has not always been true, nor will it be true in the future. The Earth’s spins around like a top. The Earth’s axis traces out a circle every 26,000 years. We call this the precession of the axil tilt or obliquity. It is also known as the precession of the equinoxes because the celestial equator and the position of the equinoxes, are slowly moving relative to the path of the Sun (ecliptic) and against the background of stars (celestial sphere). In 12,000 years, the Earth’s axis will point toward the star Vega.
• One more thing, as the Earth’s axis traces out the precision circle, there is a little additional wobble in it called nutation due to the gravitational pull of the Earth’s Moon.

By the way, are you wondering if other planets have seasons? The answer is yes and no.

• Mercury, Venus, and Jupiter have very little or no tilt, so they don’t experience (much) seasonal variation.
• Mars, Saturn, and Neptune are tilted much the same as the Earth and they go through four seasons like the Earth.
• Uranus and our favorite ex-planet now dwarf planet Pluto are tilted over on their sides. So, they have extreme seasons where one pole is pointing toward the Sun and one pole away from the Sun during their summers and winters.

Well that’s all about seasons. Hopefully, it wasn’t too complicated. In the end, it is all because the Earth is tilted.