- Seasons
- Solstices and equinoxes
- Precession


[SLIDE 1]

What causes seasons? Seasons are affected by the position of the Earth in relationship to the Sun. The Earth is pointed towards Polaris (the North Star) throughout the year.

The orientation of the Earth's axis in relation to the Sun changes as it orbits the Sun. 

For example, the Northern Hemisphere is tipped towards the Sun in June and away from the Sun in December. The opposite is true for the Southern Hemisphere. That is why these hemispheres have opposite seasons. Changing amounts of sunlight gives rise to seasons.

When the angle of the Earth to the Sun is steeper, more sunlight occurs, leading to warmer temperatures. When the angle is more shallow, less sunlight occurs, leading to cooler temperatures.


[SLIDE 2]

To help us mark changing seasons, we define four specific times of the year. Equinoxes and solstices help mark the progression of seasons. 

June solstice, or summer solstice, occurs when the Northern Hemisphere is tipped *towards* the Sun and gets the most sunlight. This happens on June 21st.

December solstice, or winter solstice, occurs when the Northern Hemisphere is tipped *away* from the Sun and gets the least sunlight. This happens on December 21st.

March equinox, also known as spring or vernal equinox, occurs when the Northern Hemisphere goes from being tipped away from the Sun to slightly *towards* the sun. This happens on March 21st.

September equinox, or fall or autumnal equinox, occurs when the Northern Hemisphere first tips *away* from the Sun. This happens on Sept 22nd.

The Sun rises precisely due east and sets precisely due west only on the days of the March and September equinoxes.

Seasons also vary by latitude. For example, Vermont has a much longer summer day and a much longer winter night than Florida does. At high latitudes, the summer Sun remains above the horizon all day long.


[SLIDE 3]

How does the orientation of the Earth's axis change over time? While we maintain our solstice and equinoxes on the same dates, the constellations appear differently over time. This is because of a change in orientation in the Earth's axis known as precession. 

Precession is the gradual wobble altering the orientation of the Earth's axis in space. Precession occurs with rotating objects. For example, if you spin a top, you will notice that while the top spins rapidly, the axis sweeps out a circle at a slower rate. Gravity pulls the object downward but does not succeed in pulling it over, just slowing it down. The same thing happens with the Earth except at a much slower rate. Each cycle of an Earth's precession is estimated to last 26,000 years, which gradually changes the direction in which the axis points in space.

The tilt of the Earth's axis stays at 23 1/2 degrees, however, and therefore seasons are not affected. This change in the orientation of the Earth's axis changes how we see constellations.