Posts Tagged ‘ellipse’

Aphelion

When something is orbiting the Sun, this is the point of the orbit that is furthest from the Sun. It applies to anything in solar orbit – planet, comet, minor planet, dust particle …

For the Earth, aphelion is around July 4th, when the Northern Hemisphere is in summer.  The word aphelion derives from the Greek words, apo meaning away, off, apart and Helios (the Greek god of the sun).

The reason why the this occurs is because orbits are elliptical and not circular. Kepler realised this and published the information in 1609. An ellipse has two foci which I suppose can be regarded as the equivalent of the centre of a circle and are used to construct the ellipse. In terms of an orbit, the Sun sits at one of the foci therefore as a body orbits the Sun it will have a varying distance.

Illustration of aphelion and perihelion

Illustration of aphelion and perihelion

There is a further explanation and animation at http://www.windows2universe.org/physical_science/physics/mechanics/orbit/perihelion_aphelion.html

 

Aberration

Aberration of light is the apparent displacement of a star from it’s true position in the sky. It is caused by a combination of the motion of the Earth in orbit round the Sun (about 30 km per sec) and the finite velocity of light (299,792.5 km per sec or , if you prefer imperial units, 186,252.5 miles per second). The rotation of the Earth also gives rise to the aberration of starlight.

To understand aberration, we need to start off with a simple easy to understand example from the familiar world around us. Imagine you are in a parked car and you look out of the window and the falling rain. Imagine that there is no wind so the rain is falling vertically. As the driver pulls away and picks up speed to say 30mph, you notice that the rain is no longer falling vertically. Actually it is, but you are moving forwards, past the raindrops, thus greating the illusion that the rain is falling diagonally, slanting towards the back of the car.

OK, so back to the starlight. The Earth is moving forwards through space and, despite the high speed at which light travels, the starlight we see effectively is slanting backwards compared to the direction of movement of the Earth in its orbit. But the Earth moves in an ellipse round the sun so the direction of ‘slant’ of the light changes too. The net result is that if the precise position of a star is recorded throughout the year, it will be seen to describe a small ellipse around its ‘true’ position … the ‘true’ position being where the star would have been seen had the Earth been stationary.

There is also a very much smaller daily effect caused by the rotation of the Earth. This is called diurnal aberration.

The maximum displacement is 20.5 seconds of arc. This number is called the constant of aberration.  For a much more thorough treatment, including a discussion of relativity and aberation, click here.

Switch to our mobile site