Seasons

Demonstrate an understanding that there are seasonal variations in the rising and setting of the Sun

Above the Equator is the Northern Hemisphere; below it is the Southern Hemisphere. Imagine if the Equator is projected into space. This would be called the celestial equator.

The path the Sun appears to take over the course of a year is called the ecliptic. This is the path of the Earth's orbit. We cannot see the stars in the day but we can tell where they would be if the Sun where not there.

The Earth's axis is titled at an angle of 23.5 (22.26) degrees to the ecliptic.

Every day the Sun appears to move 1 degree eastwards. This is because there are 360 degrees in a circle and 365 days in a year.

Between December and June the Sun is in position above the celestial equator and the Northern Hemisphere receives more daylight. Between June and December the Sun is below the celestial equator and the Southern Hemisphere receives more daylight.

During the summer months in the Northern Hemisphere the North Pole receives sunlight for 6 months and the Sun does not set. During the winter there is darkness for 6 months. The opposite is happening at the South Pole in this time.

Equinox is the time when day and night are of equal length. Solstice is when day is at its shortest or longest.

Let's look at some 'lines' to understand this. The equator is an imaginary line around the widest point of the Earth.

There are two other lines of latitude which we need to learn. These are the Tropics of Capricorn and Cancer. They are named after these constellations as they are named after the constellation the Sun is in at different parts of the day.

Around March 21st and September 21st the Sun passes over the celestial equator. If you were standing on the Equator the Sun would be directly above your head at midday.

Around 21st December the Sun is at its furthest point southwards on the Ecliptic. If you were standing on the Tropic of Capricorn you would see the Sun above you at midday.

Around 21st June the Sun is at its furthest point northwards on the Ecliptic. If you were standing on the Tropic of Cancer you would see the Sun above you at midday.

The equinox does not stay at the same time every year. Earth's axis wobbles slightly like a spinning top. This means that the Vernal Equinox is getting slightly earlier each year. This is called "Precession of the Equinoxes". The point at which the Sun crosses the ecliptic at the Vernal Equinox is called the "First point of Aries" and since our ancients observed it in Aries, it has shifted into the constellation of Pisces and in the not distant future will occur in Aquarius.


 

Image New Window

Data

 

± Date

Northern Hemisphere

Southern Hemisphere

Vernal EQUINOX

21/3

Spring

Autumn

Summer SOLSTICE

21/6

Summer

Winter

Autumnal EQUINOX

21 /9

Autumn

Spring

Winter SOLSTICE

21 /12

Winter

Summer


Links New Window

Zoom Astronomy Seasons & Tilt

Bad Astronomy About the seasons

Harvard-Smithsonian Center for Astrophysics High Energy Astrophysics Division Activities about Earth's orbit/seasons etc

Bad Astronomy When Seasons Start

U.S. Naval Observatory AAD Earth's Seasons, Equinoxes, Solstices, Perihelion, and Aphelion 1992-2020 Data

Interactive

Interactive Equinox & Solstice

Questions

  • Explain why Earth has seasons
  • Describe the terms 'Equinox' and 'Solstice'