As we live on the Earth and can study it directly, we know more about our home planet than any other. Because ancient civilizations had a very limited understanding of Earth’s place in the universe and the solar system, they experienced great difficulty explaining events such as the day and night cycle, tides, eclipses and seasons. 

These events are impossible to explain scientifically without first understanding that the Earth is in constant orbit around the Sun, as described in the heliocentric (Sun-centred) model of the solar system.

Although it is often thought that it is warmer in summer because the Earth is simply closer to the Sun, our proximity to the Sun has little affect on the seasons. Rather, the seasons are caused because the Earth’s axis of rotation is tilted at an angle of 23.5 degrees from the orbital plane while we circle the Sun. Because of this tilt, the ecliptic and the celestial equator are inclined 23.5 degrees from each other. The progression of the Sun along the ecliptic will cause it to be located north of the celestial equator for half the year and south for the other half of the year. The northern hemisphere experiences the warm temperatures of summer while the Sun is north of the celestial equator. This is because the northern hemisphere is tilted towards the Sun at this time, and as a result is exposed to the Sun’s light for a longer period of time over one complete axial rotation (one day) during the summer. This leads to earlier sunrises and later sunsets, and because the Sun is in the sky for a longer period of time, we receive more heat from the Sun over the course of a day in the summer. The 23.5 degree tilt also causes the Sun to follow a higher path in the sky, resulting in the northern hemisphere receiving more direct sunlight and therefore providing the warmer temperatures.

The point along the ecliptic when the Sun is furthest north of the celestial equator (at its highest point in the sky) is called the summer solstice (usually 21 June) and is the longest day of the year in the northern hemisphere. Winter in the northern hemisphere occurs for the exact opposite reason: the Sun is south of the celestial equator and as a result the days are shorter in duration and less energy is received because the Sun is far lower in the sky. The winter solistice (usually December 21) occurs when the Sun is furthest south of the ecliptic. It should be noted that when it is winter in the northern hemisphere, it is summer in the southern hemisphere with longer days of more direct sunlight. In addition to the solstices, which officially mark the first days of summer and winter, there are also two equinoxes. These are the two points where the ecliptic crosses the celestial equator. The vernal equinox officially marks the first day of spring in the northern hemisphere (autumn in the south) and is when the Sun crosses the celestial equator moving north. The autumnal equinox occurs when the Sun crosses the same plane traveling south, and is officially the first day of autumn (spring in the south). On these two days, the Sun is in the sky for 12 hours; these are the only days of the year where day and night are of equal duration.