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What Is Autumnal Equinox?

The autumnal equinox, a traditionally observed event, inaugurates the first day of fall. Since this event occurs when the Earth's north pole and south pole rest at an equal position from the sun, the autumnal equinox actually heralds a range of different weather conditions for different parts of the Earth. During the autumnal equinox, the sun's subsolar point is at the Earth's equator, and this is where the sun can be seen directly overhead.

Seasonal Transition
- Sandwiched between the summer and winter solstices, the autumnal equinox announces a distinct season of the year. While this seasonal transition occurs late in September in the Earth's Northern Hemisphere, it occurs late in March in the Southern Hemisphere. However, in both locations, autumn is preceded by summer and followed by winter. During autumn months, conditions become chiller as that part of the Earth is exposed to less sunlight.
Day and Night
- Two Latin words, "æquus" and "nox," produce the term "equinox". The former word means "equal," while the latter means "night". As the equinox occurs, day and night each last for roughly 12 hours. In anticipation of the autumnal equinox, days begin to shorten by three minutes a day once summer has reached its peak, late in June. After the autumnal equinox, nights begin to lengthen.
Misconceptions
- Forces of the Earth's gravity are not noticeably affected during an equinox, and eggs are just as difficult to balance on this day as they are any other day of the year, according to astronomer Phil Plait.
  
  Another misconception of the autumnal equinox is that day and night will each be precisely 12 hours in length. Because of factors as diverse as an observer's distance from the Earth's equator, the dispersion of light in the atmosphere and the presence of visual obstructions, the rising and setting of the sun on the Earth's horizon occurs at different times for different observers.
Fall Harvest
- Traditionally, the autumnal equinox is celebrated as a point of the year during which a final harvest is reaped, before the cold months ensue. Jennifer Trainer Thompson, author of "The Joy of Family Traditions," remarks, "Although the autumnal equinox falls in September and not November, the connection to the harvest has many parallels to our modern Thanksgiving."
Differences from Vernal Equinox
- In certain respects, the vernal equinox is merely the reverse of the autumnal equinox. As the Northern Hemisphere receives the vernal equinox in March, the Southern Hemisphere simultaneously experiences the autumnal equinox. During a vernal equinox in the Northern Hemisphere, the sun expresses a northerly motion over the celestial equator, rather than a southerly direction, as is the case in the autumnal equinox. In both cases, however, days and nights are of a roughly equal length.

Astronomy