When Does Fall Start?

The question, “When does fall start?” has five possible answers, depending on whether you are in the Northern or Southern Hemisphere, you are speaking astronomically or meteorologically, or you live in one of the Asian countries that use a lunisolar calendar.

We would not have seasons if the Earth was not tilted on its axis at an angle of about 23.45 degrees. So at different times of the year, different portions of the planet are more directly exposed to the Sun’s rays. This is why the Northern and Southern Hemispheres have opposite seasons. 

Meteorological fall starts on September 1st in the Northern Hemisphere and March 1st in the Southern Hemisphere. Astronomical fall starts between September 22nd and 23rd in the northern Hemisphere and between March 20th and 21st in the Southern Hemisphere. Using the lunisolar calendar, fall starts around August 8th.

Astronomical

The Fall Equinox takes place at the exact moment when the sun is perpendicular to the equator. For example, on September 22, 2010, the Fall Equinox happens at 11:09 pm EDT, for the Northern Hemisphere. In 2011, it would be at 5:04 am EDT, and in 2012 at 10:49 am EDT. For the Southern Hemisphere, the Fall Equinox happens in March.

The reason the seasons are reversed is the tilt of the earth on its axis. On the fall and spring equinoxes, the sun rises exactly east and sets exactly west. In other words, on those two days, you can find due east and due west. Ancient people used this information to set up their calendars.  

Meteorological

As you can see, the date of the astronomical first day of fall changes every year.  Meteorologists to keep records when the seasons start at the beginning of a month and end at the close.  Seasons are mostly determined by temperature.

The old Roman calendar started the year on the first day of March, and that was the beginning of spring, with each season lasting three months. The Societas Meteorologica Palatina, in 1780, defined a season as being three whole months. Since that time, meteorologists have used that definition.

So, for meteorologists, fall starts on September 1st. Also, it is easier to calculate and compare averages, such as temperature, if the time span involved is precise.  

Lunisolar Calendar

The Chinese developed a calendar based on moon phases and the time of the solar year. The seasons are based on 24 “solar terms” and start between the equinoxes and solstices. These 24 solar terms are points on the ecliptic (path of the sun) and occur every 15 degrees. Since the Sun’s speed changes according to how far it is away from the Earth, the terms vary a little all through the year. So, fall would start around August 8th for some people in China, Korea, Vietnam, and Japan.