# Why both sunrise and sunset use the position of the sun's upper limb?

It is my understanding, that astronomical moments of both rise and set use the relative position of background object's upper limb to the foreground object's horizon.

When were these definitions formulated?

Is this done purely because sunrise and sunset are the main examples of such events and it is easier for the naked eye to tell when sun is completely below horizon?

Wouldn't this offset the symmetry of rotations (since the "day" is always longer than the "night" by the duration it takes the background object to move fully below the foreground horizon)?

• I would guess that then the upper limb disappears the light simply disappears even if visibility is poor. If you want to determine when the lower limb touches, it requires much better imaging perhaps. e.g. 01:30 here: youtu.be/lwus2nqU0SY Compare to the funny business at the bottom of the moon making it difficult to determine when it last touches the horizon here youtu.be/LEH0WmCZTAo
– uhoh
Sep 9 '20 at 4:44
• Ya, that might in fact be right and my guess is wrong!
– uhoh
Sep 9 '20 at 7:06

When were these definitions formulated?

Multiple religions have rather strict requirements based on sunrise / sunset, for example, when one must start or can stop fasting, or when exactly to sacrifice a lamb (or a human in some religions). The definition has very deep roots.

Is this done purely because sunrise and sunset are the main examples of such events and it is easier for the naked eye to tell when sun is completely below horizon?

Yes.

Wouldn't this offset the symmetry of rotations (since the "day" is always longer than the "night" by the duration it takes the background object to move fully below the foreground horizon)?

Daytime is not longer than nighttime in the middle of winter, in places that do have winter. The definition of sunrise and sunset does make daytime be a bit longer than nighttime on the equinoxes.

Suppose sunrise was instead defined as the time at which the entire Sun first appears above the horizon and sunset was instead defined as the time at which the first bit of the Sun drops below the horizon. Even with these extreme definitions, daytime would still last longer than nighttime on the equinoxes thanks to atmospheric refraction.

• The skew towards longer day than night at equinoxes is also - and even more - by the effect of refraction, that is the sun is physically already below the horizon but the atmosphere still bends the sun rays towards the observer. This effect is around arc minutes, thus bigger than the sun's diameter. Sep 9 '20 at 10:20
• @planetmaker - That is exactly what the last paragraph in the answer addresses. Sep 9 '20 at 18:49
• timeanddate.com/astronomy/refraction.html has a nice picture showing the apparent sun and astronomical sun, and states that the difference may be "several minutes". It would be nice to have some kind of typical value or range here. Sep 10 '20 at 23:00