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The September Equinox or Southward Equinox
autumnal (autumn) or fall equinox for Northern Hemisphere / vernal equinox or spring equinox for Southern Hemisphere
For the Northern Hemisphere the September Equinox is the autumnal or fall equinox, while in the Southern Hemisphere it is the Vernal Equinox or Spring Equinox.
There are two equinoxes every year
There are two equinoxes every year; one in September and one during March. At the equinox the Sun passes directly over the equator and the length of day and night is approximately equal all over the planet On the September Equinox the Sun is passing southwards from the Northern Hemisphere into the Southern Hemisphere and seasons become opposite between the two hemisphere’s. The change of temperature is especially prevalent above and below the Tropic Zone: (i.e. above the Tropic of Cancer and below the Tropic of Capricorn) .After the September Equinox the Zones above the Tropic of Cancer begin to get significant colder and zones below the Tropic of Capricorn become significant warmer.
At the time of the equinox the sun rises directly in the east and sets directly in the west. Before the September Equinox/Southward Equinox, the sun does not rise exactly in the east, nor set directly in the west, instead before the September Equinox the Sun rises and sets increasingly more and more to the north, and after the September Equinox, the Sun rises increasingly more to the south.
Earth’s axil tilt is 23.4393° to the plane of its orbit
To the highest accuracy Earth’s axil tilt is 23.4393° to the plane of its orbit around the Sun. However it is often rounded up to 23.44° (or) 23.5° and this is usually sufficient enough to answer questions in exams. Just watch out for the number of decimal places required in the answer. On the day of the equinoxes the orbit of Earth is perpendicular (at 90 degrees) to the Sun and appears from Space as shown in the diagram. As viewed from the equator, the Sun is directly overhead.
Why do the equinoxes not always occur on the same day every year?
The Earth takes approximately 365.25 days to orbit the Sun. Over time the extra quarter day adds up, so that is why an extra day was introduced into the calendar every fourth year – the leap year having 366 days. If we didn’t have a leap year the dates would gradually drift out of synch with the seasons. The same principle occurs with the equinoxes and they do not occur at precisely the same time, instead they generally occur approximately 6 hours later each year, with a jump back of a day every leap year. The Southward/September Equinox usually occurs on the 22nd September or the 23rd September; however although it rarely occurs, it can also occur on the 24th September.