8. Why do we have seasons?

Image Credit: Kristian H. Resset via Wikimedia Commons.

In the last chapter, I described how an astronomer living 5000 years ago could have measured the length of the year by tracking the path of the Sun through one cycle of the seasons.  The story wasn’t set at any specific location because we don’t know who first did this.  It’s likely that people from different parts of the world figured it out independently.

By around 5000 years ago, seasons had become enormously significant to many societies.  The year became as important as the month, if not more so.  Every new moon was observed.  But a new moon that occurred close to a solstices or equinox was celebrated.  Monuments were aligned with the cardinal directions or with the Sun on a solstice.

This does not mean people figured out, 5000 years ago, why we have seasons.  In fact, most people were wrong about this even 500 years ago.

To figure out why we have seasons, we had to observe how the seasons change not just from month to month but also from place to place.  Our perspective had to go from local to global.

Over the centuries, different parts of the world came in contact with each other through trade and conquest.  If you lived in a major city, you could meet people from distant lands.  They could tell you that some places have long days and short nights followed by short days and long nights while other places have equally long days and nights throughout the year.  And that the Sun is directly overhead on certain days in some places but that this never happens in other places.

You could incorporate all this information into a model of the Earth and the Sun and use it to explain seasons.  Chances are, your Earth would have been flat.  The mythologies of ancient Mesopotamia, Egypt, India, China, and Greece all depict a flat Earth.

Your Sun would orbit your flat Earth once a day.  And it would also slowly drift north or south.  It would be furthest to the north or the south on the solstices and halfway between these points on the equinoxes.

In fact, evidence that the Earth was round, not flat, was all around us.  As Aristotle noted:

Our observation of the stars make[s] it evident, not only that the Earth is circular, but also that it is a circle of no great size.  For quite a small change of position on our part to south or north causes a manifest alteration of the horizon.  There is much change, I mean, in the stars which are overhead, and the stars seen are different, as one moves northward or southward.  Indeed there are some stars seen in Egypt and in the neighborhood of Cyprus that are not seen in the northerly regions; and stars, which in the north are never beyond the range of observation, in those regions rise and set.

Aristotle, On the Heavens, Book II, Chapter 14, 297b 26-298a 5 (Oxford trans., pp. 488-489)

In Greece, the transition from flat Earth to round Earth happened around 2500 years ago.  According to Steven Weinberg, earlier philosophers could not accept a spherical Earth because they thought travelers would fall off.

The real challenge is often not what you have to learn but what you have to unlearn.  If you’re sure the Earth is flat, no amount of data indicating that it is round will be enough to change your mind.

While Aristotle was right about the approximate shape of the Earth, he and most of the other ancient Greeks were still wrong about why we have seasons.  They still believed the Sun orbits the Earth.

In the geocentric (Earth-centered) model, the Sun slowly drifts towards the north for half the year and towards the south for the other half of the year.  The furthest north it gets is 23.4 degrees North of the celestial equator (the Tropic of Cancer).  This takes place on June 20th or 21st and is called the summer solstice in the Northern Hemisphere.  Then the Sun moves south until it is on the celestial equator.  This is on the autumnal equinox in the Northern Hemisphere which takes place on September 22nd or 23rd.  The Sun continues moving south until it is 23.4 degrees south of the celestial equator (the Tropic of Capricorn).  This is the winter solstice in the Northern Hemisphere and takes place on December 21st or 22nd.  Then the Sun heads northward.  It will cross the celestial equator again around March 20th, which is the vernal (spring) equinox in the Northern Hemisphere, on the way to the next summer solstice.

Seasons in the Southern Hemisphere are the opposite of those in the Northern Hemisphere.  Northern summer solstice is southern winter solstice and northern autumnal equinox is southern vernal equinox.

Arguably, the greatest revolution in the history of science was the shift from the geocentric model to the heliocentric (Sun-centered) model of the solar system around 400 years ago.

It is the Earth that moves around the Sun, not the other way around.

Today, we explain seasons by giving the Earth a tilt.  The Earth’s axis of rotation is tilted by 23.4 degrees relative to the plane in which it orbits the Sun.  When your hemisphere (Northern or Southern) is tilted directly towards the Sun, the Sun appears higher in the sky than when it’s tilted away from the Sun.  This is shown in the animation below.  If it makes you dizzy, don’t stare at it.  You can focus on the figure below it which shows snapshots of the Earth on the two solstices.

Now that we have telescopes and satellites, the idea of a motionless Earth at the center of the universe might seem absurd.  But the geocentric model explained almost everything you can see with the naked eye just as well as the heliocentric model did.

If you were an ancient astronomer, would there have been any way for you tell which model is right?