Sun Farthest from Earth

Sun is far away and still its hot summer in the northern hemisphere. This is the question many have, relating to the distance to the Sun and seasons on earth. The seasons on earth is due to the tilt of the earth at 23.5 degrees. Due to this tilt northern and southern hemispheres are exposed to the Sun during certain months of the year. While Northern hemisphere is exposed to more sun during March to September, the Southern hemisphere will be going through winter and situation reverses after September. From September to March Southern hemisphere is exposed to more Sun and it will be Summer in southern hemisphere while northern hemisphere will be experiencing winter. This is how seasons change on earth.

Earth Tilt and Seasons Credit:CC

Earth makes one full revolution of the Sun in approx 365.256 days. In this orbit around the Sun there are two points where the Sun and earth are closest, called Perihelion, and the other point is the farthest distance between Sun and earth and this point is called Aphelion. The two points Perihelion and Aphelion exists because the orbit of earth around the Sun is not a circular one, it orbits in an elliptical orbit but the difference is very small at an eccentricity of 0.0167. This small elliptical orbit makes the earth”s perihelion distance close to 147 million kilometres and aphelion distance close to 152 million kilometres.

Earth elliptical orbit Perihelion and Aphelion  Credit: ABAA

Earth reaches perihelion on January 5^th and aphelion on July 4^th. During Perihelion the Sun is in the constellation of Sagittarius and earth will be moving around the Sun at a velocity of 1,08,900km/h and has an angular diameter of 32m32s of arc. During Aphelion on July 4^th Sun will be in the constellation of Gemini and earth will be moving around the Sun at a velocity of 1,07,208km/h and will have angular diameter of 31m28s of arc.

Sun Angular size at Aphelion and Perihelion  Credit: ABAA

Happy Observing and Clear Skies.

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Earth and Sun Closest

Sun so close yet so cold...Yes Today Sun and earth will be closest to each other at a distance of 14,70,99,586 km. This point of any object closest to Sun is called Perihelion. The reason for the cold in northern hemisphere is due to the orientation of earth's axis and as you can see in the illustration, the Sun is moved to southern hemisphere where they are experiencing Summer now. 

Since earth's orbit is almost circular, at farthest point from Sun which occurs on July 6th the distance will be 15,20,96,155 km. This farthest point from Sun is called Aphelion. The angular size of the Sun which we measure in degrees at perihelion is 0.542018074degrees. And as we know the orbit is almost circular the difference in angular diameter of Sun from Perihelion to Aphelion is very small. At Aphelion the angular diameter is 0.5243709741degrees. 

 If we compare this to Mercury, the perihelion distance is 46,000,000 and this gives the angular diameter of Sun Seen from Mercury at 1.73434325degrees. The Aphelion distance of Mercury is 69,820,000 and this gives the angular diameter of Sun as seen from Mercury at 1.14247784degrees, almost half a degree difference. You can compare this to earth and Sun.

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Continuing the discussion on the topic of Earth's orbit

Continuing the discussion on the topic of Earth's orbit, that took place at ABAA yesterday, Jayanth has sent this post for further discussion.

Table gives the calculations of earths orbit assuming same is elliptical. If this is verified by observations (Red marked values), then it a proof that the orbit of earth is not circular but elliptical.

Observational proof of Earth's Elliptical orbit
Planet	Earth
Mean period	365.25
Earth year	1
Eccentricity	0.017
Mean AU	1
AU @ Perigee	0.983
AU @ Apogee	1.017
Year Perigee	0.974609
Year Apogee	1.025608
Earth Year Perigee	355.9758
Earth Year Apogee	374.6033
Angular velocity of earth due to rotation / day	0.2500000
Angular Velocity Perigee / day in degrees	1.011305
Angular Velocity Apogee / day in degrees	0.961017
Angular velocity Perigee translated to time sec	242.7131
Angular velocity Apogee translated to time sec	230.644
Time difference in sky revolving due to earth's elliptical rotationSec	12.06915
Angular velocity if earth orbit is circular	0.985626
Angular velocity translated to time sec Circular orbit	236.5503
Mean circular time - Perigee time	-6.162805
Mean circular time - Apogee time	5.906347

How to observe ?

Fix a telescope pointed to a bright star of say 3rd magnitude. Lock the movements of telescope. Use a highly accurate stop watch say +/- 0.1 sec per day which is easily available. Align cross hair position on eye piece in North - South and East-West direction. Start stop watch once star centers and is on North - South cross hair. Leave the setup till next day without disturbance (Better inside the room to take care of the weather, wind, heat..............). Next day at the same time observe the same star drifting in the eye piece. Stop the stop watch once the star is on North - South Cross hair. Note the time.

It is expected to have a timing of 86400-236.5503 = 86163.4497 sec if circular.

If elliptical, depending on the position of the earth around the sun, time will vary. For Perigee it should be 86400-242.7131 = 86157.2869 sec and for Apogee it should be 86400-230.644 = 86169.356 sec

In reality, we should go the other way by observational data over the year, work back to find the Eccentricity of the orbit......... ;-))

Happy Observation

Regards

Jayanth.B

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