Iranians across the world celebrate first day of winter. First day of winter is also longest night for northern hemisphere which in Iran Called Yalda or birth of sun.
Because of the tilt in the Earth’s axis of rotation ( 23 degree), the sun appears to rise and fall in our sky over the course of a year. It’s not the sunitself moving, but the Earth moving relative to the sun.
On other word the northern hemisphere spend more time in its own shadow during winter and the southern hemisphere is more exposed to sun. At the same time north pole is completely deprived from sun rays (night) while in the south pole sun covers the entire Antarctica for months. The long night for people of the northern hemisphere means winter and for the southern hemisphere it means summer.Since then length of days gradually increase and apparent sun,s movement indicates that sun moves northward on sky.
On Dec. 21, the sun stops moving southward, pauses, and then starts moving northward. This pause is called the "solstice," from the Latin words "sol" for "sun" and "sisto" for "stop." Similarly, on June 21 the sun stops moving northward and starts moving southward.
That's why Yalda is celebrated as Birth of Sun among Iranian communities.
Check the following video clip which I made using Google Earth to explain winter solstice.
Earth rotation and winter solstice
Because of the tilt in the Earth’s axis of rotation ( 23 degree), the sun appears to rise and fall in our sky over the course of a year. It’s not the sunitself moving, but the Earth moving relative to the sun.
On other word the northern hemisphere spend more time in its own shadow during winter and the southern hemisphere is more exposed to sun. At the same time north pole is completely deprived from sun rays (night) while in the south pole sun covers the entire Antarctica for months. The long night for people of the northern hemisphere means winter and for the southern hemisphere it means summer.Since then length of days gradually increase and apparent sun,s movement indicates that sun moves northward on sky.
On Dec. 21, the sun stops moving southward, pauses, and then starts moving northward. This pause is called the "solstice," from the Latin words "sol" for "sun" and "sisto" for "stop." Similarly, on June 21 the sun stops moving northward and starts moving southward.
That's why Yalda is celebrated as Birth of Sun among Iranian communities.
Check the following video clip which I made using Google Earth to explain winter solstice.
Earth rotation and winter solstice
