Why are there more than 12 hours of daylight on the Equinoxes?

 

#Toronto enjoyed 12h 01m of daylight today, March 17, 2025. So, why wasn't today the #Equinox?

 

The Equinox is not until March 20th when Toronto will have 12h 10m of daylight – not exactly “equal day, equal night” - however, on the Equinoxes there are about 12 hours between the times at which the centre of the actual sun’s disk aligns with the horizons (i.e. at 0° altitude).

Now, there are two reasons why we actually have more than 12 hours of daylight on the Equinox…

 

1) the official times of #sunrise and #sunset are when the top edge (not the centre) of the sun’s disc appears on the horizon, and

 

2) the refraction of the sun's rays through the atmosphere – yes, light bends around the earth.

 

See diagram below.

 

When we first see the sun at sunrise, or when we last see it slip below the horizon at sunset, the sun is actually below the horizon. The following explains…

 

The semi-diameter of the sun is about 16 minutes of arc and average refraction accounts for about 34 minutes of arc. So, since the true sun’s centre is about 50 minutes of arc (i.e. about 1.5 sun diameters) below the horizon when we see a sunrise or sunset, the overall length of daylight increases.