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 and, in cultures which used cyclic calendars based on the winter solstice, the "year as reborn" was celebrated with reference to life-death-rebirth deities or "new beginnings" such as Hogmanay's "redding", a New Year cleaning tradition. Also "reversal" is yet another frequent theme, as in Saturnalia's slave and master reversals. Iranian people celebrate the night of the Northern Hemisphere's winter solstice as, "Yalda night", which is known to be the "longest and darkest night of the year". In this night all the family gather together, usually at the house of the oldest, and celebrate it by eating, drinking and reading poems