RYA-tips #12 – Equinox literally means equal night, but is that correct?

Zeilen is een combinatie van wetenschap en kunst. En voor wie daar meer over wilt weten schrijft Albert De Nijs, instructeur bij de De Zeezeilers van Marken wekelijks een rubriek met tips&trics van de Royal Yachting Association.

Around 21 September the Sun crosses the celestial equator on its way south. All over the world (except at the poles) the Sun rises due East, sets due West and there is 12 hours daylight and 12 hours night-time.

Generally speaking that is correct, but if you look in the nautical almanac for astro navigation you’ll find that the time between sunrise and sunset is slightly more than 12 hrs. There are two reasons for this. 

At the equinox the centre of the Sun is 12 hrs above/below the true horizon. Sunrise is defined when the upper limb shows above the horizon, sunset when the upper limb dips below the horizon. The diameter of the Sun is around 32 minutes of arc, this ½ diameter adds a few minutes to the daytime.

The other reason is refraction. When the Sun is low on the horizon the rays of light travel a long way through the atmosphere and are refracted. In fact, the Sun is already below the horizon when we can still see the light. This adds about 6 minutes to the daytime. So, at equinox there is still about 12 hrs 8 min of daytime and 11 hrs 52 min night-time!

Albert de Nijs, Dutch Offshore Sailing Academy