Summer heat is still here in full force, but the amount of daylight we experience is definitely getting less and less with each passing day, an imminent sign of the arrival of fall. 

We officially welcome fall in two different ways on two different dates: Sept. 1 and Sept. 23. The reason there are two dates to mark the beginning of every season is because we define our seasons in two ways. Meteorological seasons are based on annual temperature cycles, while astronomical seasons are based on Earth's position in relation to the sun, according to National Geographic.

Based on meteorological seasons, fall begins on Sept. 1 and ends on Nov. 30 each year. This year, astronomical fall begins on Sept. 23, but the date for the start of astronomical seasons can vary slightly from year to year because it is based on Earth's position in relation to the sun. It takes slightly longer than one calendar year — 365.2422 days to be exact — for Earth to orbit the sun, National Geographic reports. This imprecise number means astronomical seasons don't always begin at the same time each year. Fall usually begins on Sept. 22 or 23, but it can begin as early as Sept. 21 or as late as Sept. 24.

The precise moment that fall beings this year, at 1:49 a.m. Sept. 23, is called the autumnal equinox. Fall and spring both being with equinoxes, while summer and winter begin with solstices. An equinox occurs when the sun is directly above Earth's equator, meaning about half the planet is light and half the planet is dark, according to National Geographic. A solstice occurs when the Equator reaches its farthest point north or south of the sun's path.

Less than a week after the autumnal equinox will be the September full moon. It will reach its fullest point at 4:58 a.m. Sept. 29, just before slipping below the horizon for the day, but it will appear full in the night sky the night before and after, according to the Old Farmer's Almanac. Most years, including this year, the September full moon is known as the harvest moon. The harvest moon is the full moon that occurs nearest to the autumnal equinox. While this normally occurs in September, the harvest moon can also sometimes be in October.

When the harvest moon occurs in October, the September full moon is called the corn moon because it comes at the time of year when corn is usually ready to be harvested. Other seasonally appropriate names for the September full moon that are used by some Native tribes include the autumn moon, the yellow leaf moon, the leaves turning moon, the moon of brown leaves and the falling leaves moon.

Also at the end of the month, two meteor showers begin, with the Orionids getting underway on Sept. 26 and the southern Taurids beginning just a few days later on Sept. 28. Both are long-lasting meteor showers. The Orionids will end Nov. 22, peaking the night of Oct. 20 to 21, according to the American Meteor Society. The southern Taurids won't end until Dec. 2, peaking the night of Nov. 4 into 5.

Of these two meteor showers, the Orionids is a better chance to catch a glimpse of a shooting star. It generally produces between 10 and 20 meteors an hour, making it a medium-strength meteor shower, according to the meteor society. The southern Taurids does not usually produce more than five meteors an hour. However, the southern Taurids are sometimes associated with increased fireball activity.