Fall 2024 is almost officially here, and with it cooler temps, changing leaves and pumpkin spice everything. Sept. 22 marks the official first day of fall this year, but that's not always the case. The first day of fall and spring do not occur on the same days every year. Fall 2023, for instance, began on Sept. 23.

The start of fall and spring is determined by an astronomical phenomenon called an equinox, when the sun is positioned exactly above the Earth's equator. 

Jose Daniel Castro-Cisneros, a graduate student in the University of Arizona Department of Physics, explains how fall and spring equinoxes work.

Q: What are equinoxes and how do they work?

A: Equinoxes are a consequence of how the Earth moves around the sun. The Earth circles the sun while spinning on its own axis, similar to a spinning top. The Earth's rotation axis is tilted with respect to its orbital plane around the sun. This tilt causes the Earth to sometimes be tilted toward the sun and sometimes away from it, resulting in our seasons.

During our summer in the northern hemisphere, the Earth is tilted toward the sun, so we receive more light than the southern hemisphere. Similarly, in our winter, the northern hemisphere is tilted away from the sun.

An equinox occurs twice a year, during the transition from winter to summer and again from summer to winter. It's the moment when the Earth isn't tilted either toward or away from the sun. 

The dates of equinoxes varies slightly from year to year. This year's fall equinox will be on Sept. 22 at 5:43 a.m. Tucson time. 

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Q: How much does the duration of daylight change around the equinox?

A: Contrary to popular belief, day and night are not exactly equal on the day of the equinox. Daytime is typically slightly longer than nighttime during the equinox. In Tucson, for example, daytime is about seven minutes longer than nighttime.

The days immediately before and after the equinox see the most rapid change in daylight duration. For instance, in the days leading up to the fall equinox, daytime has been reducing by almost two minutes every day, which is the most significant change seen throughout the year.

Q: How do different cultures view or use the equinox?

A: Different cultures have used various conventions for defining seasons and calendars. The equinox, being an objective event, has been used by some cultures as a reference point. For example, the Persian calendar sets the beginning of the year at the spring equinox. This means their New Year's date may vary slightly from year to year, as it's based on this astronomical event rather than a fixed date like Jan. 1.

Q: How do astronomers and physicists use equinoxes in their work?

A: For astronomers and physicists studying celestial bodies, equinoxes serve as important reference points. They use the equinox to define the positions of celestial objects relative to the direction of Earth's rotation axis at the moment of the equinox. This helps establish a standard system for measuring the positions of objects in space.

Q: Can people observe the equinox directly?

A: While it's not straightforward, it is possible to observe effects related to the equinox. Ancient cultures realized this event by tracking the apparent motion of the sun. The sun's path across the sky changes throughout the year. In winter, it's closer to the horizon, while in summer, it's higher in the sky. At the equator, during an equinox, the sun moves in a perfectly vertical direction from geographical east to west. However, this may vary depending on latitude, and it's important to remember that the equinox is a specific moment, not a whole day.