YALDA A night to remember

Winter is coming and autumn is going out.Geographically speaking northern hemisphere will go to its shade more often and southern hemisphere will expose more to sun. Yalde celebrate by Iranians and some other nations as longest night of northern hemisphere which equal to winter solstice (first day of winter). Yalda word comes from Syriac language (a forgotten language in Lebanon  and Syria) which means birth. In first day of winter longest night will happen but since then day length gradually will increase in  a rate of a few minutes every day. So more sun light and thus it is sun's birth. Since earth travel a big curve around Sun, changes in day and night takes a month to be noticeable. While in northern hemisphere experiencing more darkness and northern pole is completely covers by earth shadow (no sunrise for six months), in southern hemisphere longest days are predominated and they have summer (in south pole no sunset for six months). In northern hemisphere days start to increase their length , in southern hemisphere days length shrink. In both hemisphere length of day and night would be equal at first day of spring (spring equinox) for northern hemisphere and first day of fall for southern hemisphere (fall equinox).In northern hemisphere day continue to increase their length until reach to first day of summer which is called summer solstice.Summer solstice in northern hemisphere  equals to first day of winter in southern hemisphere hence winter solstice. All of the above changes happens because of inclination in earth axis.

 Earth axis is an imagery line from north pole to south pole.This line makes almost 23 degree to its perpendicular rotational axis, called axis tilt.Without axis tilt there is no seasons(no temperature exchanges) and length of day and night were equal (very hot every day, no north and south poles).As the year progresses and the earth revolves around the sun, the tilt of the earth’s axis doesn't change. The axis is 23 ½ degrees away from a straight up and down line relative to the sun’s position. This means that for some of the year ( northern hemisphere summer), the North Pole is aimed toward the sun, and some of the year it is aimed away from the sun ( northern hemisphere winter). It is vice versa in the southern hemisphere.

So remember Yalda many geographical astrological changes have happened to make your planet livable and beautifully desirable. 

Summer Solstice Northern hemisphere

Winter solstice Northern hemisphere 

Longest Night Or Birth of Sun

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