Apparent Migration of the Sun
The Sun does not move, the Earth moves around the sun but to us on the Earth, it appears as if it is the Sun that moves between the Tropic of Cancer and the Tropic of Capricorn. This means that all the latitudes between the Tropic of Cancer and the Tropic of Capricorn will have the sun vertically overhead (making 90 degrees angle) twice a year, once between June and December and again between December and June. The places located beyond the two tropics never experience the overhead sun (making 90 degrees angle).
Mathematical Explanation :
Earth takes about 365 and a quarter days to travel once around the sun. Since the earth moves around the sun and we are resident on the earth, it means that the sun appears to shift in the sky about 1 degree per day.
⇒ Since, Earth moves 360 degrees around the sun in year so total number of degrees = 360 degrees
⇒ Total number of days that earth takes to move around the Sun = 365.2422 days
⇒ Degrees that it moves per day = 360 / 365.2422 = About 1 degree per day.
Northward Apparent Migration (Uttarayan) : For six month of the year, the sun appears to be moving north. This northward migration begins after December 22 and is completed on June 21, when Sun is directly overhead at 23.5 degrees North of latitude (Tropic of Cancer ). This Position of Earth (21 June) that is, when Sun is vertically over the tropic of cancer is called the Summer Solstice.
Southward Apparent Migration (Dakshinayan) : For six months of the year, the sun appears to be moving south. Thois southward migrationbegins after June 21 and is completed on December 22, when Sun is directly overheadat 23.5 degrees south of latitude. (Tropic of Capricorn ). This position of the Sun (22 December) , that is, when the sun shines vertically over the Tropic of capricorn is called the winetr solstice.
The apparent movement of the sun results in chnages in the altitude of the mid-day Sun at different times of the year, so the seasonal changes are caused according to the earth position in the elliptical orbit
Summer Solstice and Winter Solstice
Solstice is derived from two Latin words: "sol" meaning sun, and "sistere" to cause to stand still. It means "standing still of the Sun" but in a broader sense, the day of the solstice is either the longest day of the year or the shortest day of the year for any place outside of the tropics.
Summer Solstice (21st June) : The North Pole is tilted towards the Sun so the rays of the sun fall vertically or directly on the tropic of cancer 23 1/2 degrees North. The altitude of the Sun is 90 degrees at 23 1/2 degrees North.
- It is summer season in the Northern Hemishphere and it has the longest day and the shortest night on Jun 21st.The duration of daylight goes on increasing from 12 hours at the Equator to 24 hours at the North Pole.Beyond Arctic circle (66 1/2 degrees North ) duration of Day is 24 hours
- The Northern Hemisphere faces higher altitude of the Mid day sun so it is summer in the Northern Hemisphere where days are longer and nights are shorter.
- At the same time South Pole is inclined away from the Sun so the the Sun's Rays do not reach the South Polar region bryond the Antarctic circle. It is winter in the Southern hemisphere and it ha sthe shortest day and the longest night on 21st June
- The duration of Day light goes on decreasing from 12 hoours at the equator to Zero hour (Complete Darknees) at the south pole. The Southern Hemisphere faces lower altitude of the mid- day Sun so there is winter where days are shorter and Nights are longer
Winter Solstice (22nd of December) : The south pole is tilted toward the Sun so the rays of the sun fall vertically or directly on the Tropic of Capricorn (23 1/2 degrees South).The altitude of the Sun is 90 degrees at 23 1/2 degrees South.
- It is summer in the Southern hemisphere and it has the longest day and the shortest night on Decemebr 22nd.The duration of daylight goes on increasing from 12 hours at the equator to 24 hours at the South pole.Beyond Antarctic Circle 66 1/2 degrees the duration of day is 24 hours.
- The Southern Hemisphere faces higher altitude of the mid - day Sun so it is summer in the Southern hemishphere where days are longer and nights are shorter.
- At the same time, the North pole is inclined away from the sun so the Sun's ray do not reach the North Polar region beyond the Arctic circle. It is winter in the Norther hemisphere and it has the shortest day and the longest night on 22nd December.
- The duration of daylight goes on decreasing from 12 hours at the equator to zero hour (complete darknees) at the North Pole. The Northern Hemisphere faces lower altitude of the mid day sun so there is winter where days are shorter and the nights are longer.
What are Equinoxes ?
Equinox : We have two equinoxes every year one in September and other in March. Equinoxes are the astronomical event when Sun shines directly on the equator and the length of day and night is nearly equal.We have two Equinoxes in year :
- Autumnal (fall) equinox (in the northern hemisphere ) : On 23rd of September, the circle of illumination passes through both the poles making 12 hours day and 12 hours night at all places on the earth. The subsolar point crosses the equator moving southward at the September equinox.
- Spring (vernal) equinox (In the Southern Hemisphere) : On 20th March the Sun shines directly on the equator and the length of day and night is nearly equal.The subsolar point crosses the equator moving northward at the March equinox .
Summary of Daylight hours in the Northern and Southern Hemisphere :
- Longest day in the Northern Hemisphere : June 21
- Shortest day in the Northern Hemisphere : December 22
- Equal day and night in the Northern Hemisphere : March 21 and September 23
- Longest day in the southern Hemisphere : December 22
- Equal day and night in the southern Hemisphere : March 21 and September 23
Important dates to remember:
- June 21st Summer Solstice (longer days and shorter nights in the Northern Hemisphere)
- September 23rd Autumnal Equinox (equal days and equal nights in both the Hemisphere)
- December 22nd Winter Solstice (Longer days and shorter nights in the southern Hemisphere)
- March 21st Vernal Equinox (equal days and equal nights in both the Hemishpheres)
Perihelion and Aphelion
The Earth travels 939,886,400 km along it's elliptical orbit in a single revolution around the Sun but the orbit is not exactly circular, so the Sun is not centrally placed within it. As, a result the distance between the Earth and the Sun changes during one revolution.
Perihelion : On about January 3rd, the Earth is closet to the Sun (147.5 million). At this time the Earth is said to be in Perihelion. (This word comes from Greek word peri meaning " near or around " and helios means " Sun".
Aphelion : On about July 4th, the Earth is farthest from the sun (152.5 million km). At this time the Earth is said to be aphelion.
Kepler's law of planetary motion : According to the Kepler's law of planetary motion, the Earth moves fastest when it is in perihelion and slowest in Aphelion. The difference in distance between the Perihelion and Aphelion is only 5 million km, which is quite smallif we consider the average distance of about 150 million km (149.5 million km) between the Earth and the Sun.