Winter solstice one must be able to observe a change in azimuth or elevation less than or equal to about 1/60 of the angular diameter of the sun. Observing that it occurred within a two-day period is easier, requiring an observation precision of only about 1/16 of the angular diameter of the sun. Thus, many observations are of the day of the solstice rather than the instant. This is often done by observing the sunrise and sunset or using an astronomically aligned instrument that allows a ray of light to be cast on a certain point around that time. Length of day
Winter solstice axis of rotation points (axial precession) change very slowly (at the current rate it would take just under 26,000 years to make a complete circle). As the Earth follows its orbit around the Sun, the polar hemisphere that faced away from the Sun, experiencing winter, will, in half a year, face towards the Sun and experience summer. This is because the two hemispheres face opposite directions along Earth's axis, and so as one polar hemisphere experiences winter, the other experiences summer. More evident from high latitudes, a hemisphere's winter solstice occurs on the day with the shortest period of daylight