Opposite Sides & Seasons
Last Friday, the 22nd of September, was the Autumn Equinox in the Northern Hemisphere. I say the Northern Hemisphere, because seasons are opposite on both sides of the equator. The Autumn Equinox is widely acknowledged as the first day of Autumn.
What causes the Seasons?
The Earth’s Axial Tilt is the angle at which the Earth’s axis is tilted at. This is approximately 23.5º in relation to an imaginary plane (flat surface) around which it orbits the Sun, called the ecliptic. On any day of the year apart from the two equinox days, one of the hemispheres is always tilted a little more towards the sun than the other. This creates a difference in the amount of sunlight, or solar radiation, that reaches different parts of the Earth. It is this imbalance of energy or radiation that creates the Seasons.
What is an Equinox and what is a Solstice?
A solstice represents a point at which either pole faces the sun directly, for a full 24 hours. During the summer, everything North of the Arctic Circle is in permanent daylight, and experiences a midnight-sun. On either the 20th/21st June however, for the Summer Solstice, the sun does not set at all at the North Pole for a full 24 hours, and everywhere in the Northern Hemisphere experiences the ‘longest day’.
During the winter, the North Pole experiences permanent dusk/night-time, as the sun does not rise above the horizon. On the 20th/21st December, everywhere in the Northern Hemisphere experiences the ‘shortest day’ as the North Pole is pointing directly away from the sun. Everywhere in the Arctic Circle will experience a midday-darkness. This can be applied a little more locally to the UK, where the evenings are longer in the summer before it gets dark at around 9-10pm, yet shorter in the winter before it gets dark at around 4-5pm.
In between the June and December Solstices are the March and September Equinoxes. This is when the tilt of the Earth’s axis is perpendicular (90º) to the sun’s rays, as our diagram above shows. The word ‘equinox’ is derived from the Latin aequus, meaning ‘equal’, and nox, meaning ‘night’.
What does the Autumn Equinox represent?
The Autumn Equinox represents the start of Autumn as marked by the astronomical calendar and lasts until the December Solstice. According to the meteorological calendar, Autumn starts at the beginning of September and lasts until the end of November.
The most noticeable change that begins to occur in Autumn is the changing of the colours of the leaves on the trees. As temperatures begin to fall across the UK, trees begin to prepare for winter as there will not be enough light for photosynthesis to happen. So, as the days begin to get shorter, trees begin to shut down their systems of food production and Chlorophyll, a chemical inside leaves that makes them green, begins to break down. The leaves begin to turn to shades of yellow, red and brown due to other chemicals inside the leaves that remain, including Carotene, the same chemical that makes carrots orange.
This video below, produced by MinuteEarth, describes the process of why leaves change colour in the Autumn in a very fun way, so check it out, and we hope you’ve learned a little bit more about what goes on in the Autumn!
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