Seasons: when are the day and night equal in length?

The tutor shares a discovery about the seasons.

I always believed that equal night and day – twelve hours each – happens at the turn of spring and the turn of fall. Last year, however, I noticed on the environment Canada website that the first day of spring was longer than twelve hours, and wondered why.

Yesterday, March 17, I noticed the day to be exactly 12 hours long: sunrise: 7:29; sunset: 19:29. No wonder the first day of spring is longer than 12 hours, if a few days before, the day is already exactly twelve hours.

This morning I researched the reason that March 17 is exactly 12 hours. At I found out.

For the day and night to be symmetric (at the spring or fall equinox), sunrise would need to be defined as when the sun is half up – that is, when the middle line of it is on the horizon. Then, sunset would need to be the instant when half the sun has already passed over the horizon, while half remains visible.

In practice, sunrise is thought to be the instant the sun becomes visible; sunset, the instant it disappears. Therefore, the time for the sun to climb halfway above the horizon, and fall the rest of the way below it, is added to the day. Hence, the equinox – when the day and night should be exactly equal – is actually more than twelve hours.


Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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