When is the first day of autumn? It depends on whether you are referring to the astronomical or meteorological autumn.
We often talk about it beginning to feel like autumn when the nights start to draw in and temperatures start feeling cooler.
There are two separate dates which could be said to mark the start of autumn in calendars. One is defined by the Earth’s axis and orbit around the sun and the second is a fixed date which is used by meteorologists for consistent spacing and lengths of the seasons.
The meteorological autumn began on 01 September 2018 and ends on 30 November 2018.
The meteorological seasons consists of splitting the seasons into four periods made up of three months each. These seasons are split to coincide with our Gregorian calendar making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics.
By the meteorological calendar, the first day of autumn is the 1 September.
The seasons are defined as Spring (March, April, May), Summer (June, July, August), Autumn (September, October, November) and Winter (December, January, February).
This astronomical autumn began on 23 September 2018 and ends on 21 December 2018.
The astronomical calendar determines the seasons due to the 23.5 degree tilt of the Earth’s rotational axis in relation to its orbit around the sun. Both equinoxes and solstices are related to the Earth’s orbit around the sun.