Winter solstice instant of the solstice can be calculated, direct observation of the solstice by amateurs is impossible because the sun moves too slowly or appears to stand still (the meaning of "solstice"). However, by use of astronomical data tracking, the precise timing of its occurrence is now public knowledge. One cannot directly detect the precise instant of the solstice (by definition, one cannot observe that an object has stopped moving until one later observes that it has not moved further from the preceding spot, or that it has moved in the opposite direction). Further, to be precise to a single day,
Solstice in degrees (e = earth orbit eccentricity, A = earth's perihelion or sun's apogee), and the noise in the result will be about 41 hours divided by d if the eye's sharpness is taken as one arc minute. Astronomical almanacs define the solstices as the moments when the sun passes through the solstitial colure, i.e. the times when the apparent geocentric longitude of the sun is equal to 90° (summer solstice) or 270° (winter solstice). The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone. The solstices always occur