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 between June 20 and 22 and between December 20 and 23 with the 21st and 22nd being the most common dates. Using the current official IAU constellation boundaries – and taking into account the variable precession speed and the rotation of the ecliptic – the solstices shift through the constellations as follows (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.): The 687-day orbit of Mars around the Sun (almost twice that of the Earth) causes its summer and winter solstices to occur at approximately 23-month intervals. Solstice A solstice is