What is an equinox?
Throughout the year, Earth’s axis tilts slightly towards or away from the Sun, so that one hemisphere will have a longer day than the other, depending on which is more inclined towards our star. But every March and September, Earth reaches a point in its orbit where neither hemisphere is tilted towards the Sun, as it passes directly over the equator. As a result, day and night are virtually the same length – 12 hours – no matter where you are on Earth.
Despite the term equinox, which is Latin for ‘equal night’, day and night are not exactly the same globally. Variations in atmospheric temperature and pressure affect the extent of refraction of sunlight, which causes the Sun to be visible for longer than 12 hours. Technically, daytime is also longer as sunrise is the moment the upper edge of the Sun, rather than its geometric centre, becomes visible over the horizon. This is also the case for sunset, which is defined as the moment the Sun’s topmost edge disappears below the horizon.
Is the autumnal equinox always on the same day?
Although it usually falls on either thee 22 or 23 of September, in 1931 is fell on 24 September. This is because the Gregorian calendar does not always match up because the planet takes 365.25 days to orbit around the Sun, meaning the equinox day is sometimes delayed a day. The next time it will fall on 24 September will be in 2303.
Why do meteorologists say that autumn begins on 1 September?
This is purely for practical reasons. Meteorologists divide the year into four, three-month seasons so that they can make more accurate comparisons of seasonal weather from year to year.
Does the autumn equinox differ between the North and South Pole?
The people living at the South Pole will experience sunlight for the fist time in six months; the autumn equinox marks their first day of spring. Those living in the North Pole experience a very contrasting change, as they are plunged into darkness for six months.
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