Post by Zadkiel on May 27, 2016 11:45:07 GMT
At the equinoxes, all latitudes receive nearly equal amounts of daylight and darkness. Though equinox means “equal night” in Latin, both of Earth’s hemispheres get a bit more than 12 hours of daylight. On March 16 — four days before the equinox — sunrise and sunset are exactly 12 hours apart. Yet by March 20, the sun is already up for 12 hours and 9 minutes.
There are two reasons for this. One is atmospheric refraction. This optical phenomenon bends the sun’s light as it passes through Earth’s atmosphere and causes the sun to appear slightly higher in the sky than it actually is. The other is how we define sunrise and sunset. The sun appears as a disk, not a single point. Sunrise is defined as the moment the sun's upper edge appears on the horizon, while sunset doesn't occur until the sun's upper edge disappears from the horizon. Together, these factors add about 10 minutes of daylight to the equinox, depending on one’s distance from the equator.
Why, then, is the spring equinox not on 16 March, when we get exactly 12 hours of daylight? First, the date with exactly 12 hours from sunrise to sunset varies with distance from the equator. Second, the equinox occurs when the sun appears directly overhead along Earth’s equator - at zero degrees latitude - and that happens around 20 March.
There are two reasons for this. One is atmospheric refraction. This optical phenomenon bends the sun’s light as it passes through Earth’s atmosphere and causes the sun to appear slightly higher in the sky than it actually is. The other is how we define sunrise and sunset. The sun appears as a disk, not a single point. Sunrise is defined as the moment the sun's upper edge appears on the horizon, while sunset doesn't occur until the sun's upper edge disappears from the horizon. Together, these factors add about 10 minutes of daylight to the equinox, depending on one’s distance from the equator.
Why, then, is the spring equinox not on 16 March, when we get exactly 12 hours of daylight? First, the date with exactly 12 hours from sunrise to sunset varies with distance from the equator. Second, the equinox occurs when the sun appears directly overhead along Earth’s equator - at zero degrees latitude - and that happens around 20 March.
