1. Time of the Seasons
We have seasons because of the tilt of the Earth’s axis.
The Ecliptic appears tilted 23.5° from the Celestial Equator because Earth’s axis is tilted 23.5° from the vertical to the plane of its orbit
Because of this tilt the Sun appears half the year North of the Equator and half the year South of the Equator
The seasons are defined based on the location of the Sun compared to the Celestial Equator
This figure represents the earth orbiting the sun over one year. It illustrates the fact that the earth’s axis is tilted 23.5 degrees.

2. Summer On the first day of Summer:
The Sun is at its farthest distance North of the Celestial Equator (23.5° N declination). The is known as the Summer Solstice.
The Northern Hemisphere is tilted towards the Sun & we experience the longest day of the year (about June 21).
The Southern Hemisphere is tilted away from the Sun. This is the beginning of Winter and the shortest day of the year there.
Sun rise and sets at its most northerly
The word solstice comes from the Latin meaning “Sun standing” because it is at the solstices that the Sun stops moving either further North or further South and begins to turn around.
What do we experience during a typical summer day that is different from a typical winter day?
- temperatures are higher in the summer. Why?
- there are more hours of daylight in the summer. Why?
During our summer months what season is it in the Southern Hemisphere (south of the equator)?
- it would be winter during our summer. Why?

3. Why it’s hotter in the Summer and colder in the Winter
Direct versus Indirect Sunlight.
Imagine standing in front of a fire on a cold night. Your hands are cold so you extend them towards the fire palms outward. Why? You want to receive as much heat as you can from the fire. You know that you intercept more heat energy by holding your hands this way rather than holding them on edge.
The Season Lag
Usually the hottest days of the year are in July and August not in June when the Sun is at its most northerly.
It takes time for the oceans and land to heat up.
The hottest time of day isn’t at noon either but an hour or two later.
At the summer solstice, the North gets more direct sunlight
and more hours of sunlight. At the same time it is winter in the South
(less direct sunlight, fewer hours of daylight).

4. In the winter months the Sun is lower in the sky for fewer hours and the sunlight is spread over a larger area.

5. “But I thought the reason it was hotter in the summer was because the Earth was closer to the Sun in the Summer.”
If this were true then why is the Earth closest to the Sun in January and farthest from the Sun in July? and
Why does the Southern Hemisphere have seasons that are opposite that in the Northern Hemisphere?

6. Winter On the first day of Winter:
The Sun is at its farthest South of the Celestial Equator (23.5° S declination). This is known as the Winter Solstice.
The Northern Hemisphere is tilted away from the Sun & we experience the shortest day of the year (about December 21).
The Southern Hemisphere is tilted towards the Sun. This is the beginning of Summer and is the longest day of the year there.
Sun rise and set is at its most southerly.

7. Spring and Autumn
The Sun crosses the Celestial Equator heading North on the first day of Spring (the Vernal Equinox, about March 21).
It crosses the Celestial Equator again heading South on the first day of Autumn (the Autumnal Equinox, about September 21).
In both the Northern and Southern Hemisphere days and nights are of equal length

8. Summary
The rotation of the Earth on its axis determines the length of the day
The orbit of the Earth around the Sun (revolution) determines the length of the year
The tilt of the Earth with respect to the ecliptic causes the seasons.