AUSTIN (KXAN) - Despite the ongoing summer heat, we officially flip the calendar to fall on Sunday, September 22nd at 7:44 a.m. Central Daylight time. On the Autumnal Equinox, the sun is positioned right over the Earth's equator, allowing equal amounts of daylight to both the Northern and Southern Hemisphere. But why does the first day of autumn not fall on the same day every year? RELATED: The fall foliage season in Central Texas is on the way Why does the date of the Autumnal Equinox change? The equinox can fall anywhere between September 21st and 24th in a given year. The reason for the variation? It takes Earth approximately 365.25 days to rotate around the sun. A slight mismatch from the calendar year of 365 days. We make up for this quarter of a day difference by adding "leap years" to our calendar but, Earth's orbit, and consequently, it's solstices and equinoxes', do not adjust to the human-acknowledged adjustment. Why do we have an Autumnal Equinox? RELATED: This Texas city is among the ‘coziest’ in U.S., study finds The position of the sun on Earth is what determines our amount of daylight. This changes every day due to our planet's 23.5° tilt and position to the sun. On the equinoxes, the sun is positioned directly over the equator. On the solstices, the sun is directly perpendicular to the Tropic of Cancer (45°N) during N. Hemisphere summer, and the Tropic of Capricorn (45°S) during N. Hemisphere winter. If you have a topic you'd like Kristen to discuss, email her at [email protected].