Expert: Philip Stahl - 5/12/2011

Question
Question1:
If the cause for stars to twinkle is the earth's atmospheric interference, then why do plants not twinkle? So, if we travel in space, won't the stars be seen twinkling?
Question2:
If summer/winter seasons on Earth are due to changes in angle of incidence of sun's rays due to Earth's revolution around Sun, then how much does changes in the distance between Earth and Sun contribute for seasonal changes? And what is the min and max distance between earth and sun? And which part of the year Sun is nearest and farthest to Earth?

Answer
Hello,

1)Stars, first of all, cannot actually be seen, i.e. as the real, discrete objects we think we see. As one former prof once put it: "stars are too distant to be seen but too bright to be ignored". What we see when we look at a star is a *diffraction pattern* (Google!) not the star itself. Even in telescopes all we see are diffraction patterns (which in excellent quality scopes becomes smaller and smaller).

Planets, on the other hand, present discrete discs which subtend a definite angle. Because the planets shine by reflected sunlight and are seen as actual discs (in other words, the real objects not merely diffraction patterns) they don't twinkle. Stars, because of being diffraction patterns are much more susceptible to the disturbances of the atmosphere as their light is in transit to us, on the ground.

2) Changes in distance of the Earth from the Sun contribute negligibly to any seasonal changes, which are largely due to the tilt of the Earth's axis (23.5 degrees) as it moves in its orbit round the Sun. Thus, for northern hemisphere winter, the tilt is *away* from the Sun so the Sun reaches lower altitudes during the day at northerly latitudes and spends less time in the sky.

The maximum distance of Earth from Sun is defined at Earth's *aphelion* point of its orbit, and is or WILL BE (this year) 94,555,000 miles  or 152,171,522 km on July 4th.

The minimum distance occurred when Earth was closest at its *perihelion* point in orbit and was on January 3rd this year, at 91,445,000 miles  or 147,166,462 km.

Thus, as you can see, the Sun would be nearest the Earth around the beginning of winter while furthest at around the beginning of SUMMER.

Obviously then, the distance has little to do with the seasons! (Which, of course, are reversed for the Earth's southern hemisphere!)