Autumn equinox 2021

By definition, the moment of autumn equinox in the Northern Hemisphere corresponds to the precise time when the apparent geocentric longitude of the Sun is equal to 180 degrees. But, a more common understanding of this phenomenon in our society is that it happens on a certain day of September when daytime equals the duration of nighttime. In 2021, the autumn equinox will arrive on Wednesday, 22^nd of September, at 19 hours 21 minutes and 06.77 seconds UTC, respectively at 22 hours 21 minutes and 06.77 seconds (UTC+3h) which is the Current Local Time in Romania, as we use the Eastern European Summer Time (EEST). At the moment of the autumn equinox, in 2021, the geocentric latitude of the Sun equals -0.56″, the right ascension has a value of 11 hours 59 minutes and 59.985 seconds, while his declination is -0.52″. As we can see by looking at the values of geocentric latitude and right ascension, they are close to 0 and respectively to 12 hours. This is the reason why we use to say about the Sun during this moment of the autumn equinox that it is in a direction opposite to that of the vernal point. Scientifically, this statement is considered partially true and it can be generally accepted as the apparent diameter of the Sun is about 30 arcminutes. The autumn equinox in the Northern Hemisphere of the Earth corresponds to the spring equinox in the Southern Hemisphere. The calendar used nowadays in Romania is the Gregorian calendar. This time instrument was created in 1582 in order to adjust the calendar drift with respect to the succession of the seasons. Therefore, according to the Gregorian calendar, the dates of the autumn equinox will always be during 21-24 September. Actually, in most years, the autumn equinox will arrive on the 22^nd or 23^rd of September while the dates when this will occur on the 21^st or on the 24^th of September are less frequent. For example, in the year 2092, the autumn equinox will occur on the 21^st of September for the first time since the creation of the Gregorian calendar. Similarly, in 1803 the autumn equinox took place on the 24^th of September, for the first time since the creation of the Gregorian calendar. Image in full resolution is available here.

Published on: Sep 20, 2021