Explainer: Fall begins this evening, but didn’t fall already start?

That is, the beginning of astronomical fall. To meteorologists, fall began weeks ago on Sept. 1.

So what is happening?

Astronomical fall

Astronomical seasons are based on the rotation of the Earth around the sun.

Astronomical fall begins with the autumnal equinox, which is when the sun shines directly over the equator and creates nearly equal amounts of day and night, according to NASA.

Equinoxes happen twice per year due to the way the Earth’s axis is tilted as it goes around the sun.

After this point, the sun will gradually rise later and later, and set earlier and earlier for shorter days and longer nights.

Meteorological fall

By contrast, meteorological seasons are based on the calendar and annual temperature changes. In this system, fall is September through November.

According to the National Centers of Environmental Information, meteorologists break seasons down into three-month groups based on annual temperature changes and the calendar.

This way, the three months that are usually hottest are labeled summer, the three that are usually coldest are labeled winter, and the transitions between the two are fall and spring.

Why do we have both?

Astronomical seasons don’t change on the same date every year, partly because the Earth’s orbit doesn’t fit exactly into a 365-day year, and partly because the orbit isn’t a perfect circle, so seasons can be between 89 and 93 days long, the NCEI said.

So, while astronomical seasons give an accurate idea of when days will change lengths and weather will generally change, it makes it very difficult to compare climate statistics for a season from one year to another.

As a result, meteorologists observing and forecasting the weather created the meteorological seasons, tying them to the normal calendar.