Comet ISON round the Western mark

26 April 2013 by Fraser Clarke

Comet ISON on 19th April 2013

Comet ISON has made its Western turn, and will now be heading East for the next 8 months... Here's the latest image in our sequence, taken on 19th of April, showing ISON moving slowly against the background stars. You can see a small tail in our image, but unfortunately we can't quite compete with the likes of the Hubble Space Telescope when it comes to impressive images!

Comet ISON is still moving very slowly as viewed from the Earth, and you can see the stars in our image are not very trailed. If you look carefully though, you'll see they are trailed in a different direction to the last image on 1st April, indication that the comet is now moving in a different direction in the sky.

Why has that happened? Has the comet decided to go in a different direction through space? Well no, the comet is still on its parabolic orbit towards the Sun. However, as we on the Earth also move around the Sun, our aspect relative to the comet changes. Hence, it appears like the comet moves backwards and forwards in the sky -- even though it's actually just moving in one direction through space. The same things happen with planets, and this backwards and forwards motion greatly confused ancient astronomers, who assumed all the planets (and stars) moved around the Earth. In the 1500's Nicholas Copernicus realised these changes in direction could be easily explained if the Earth, along with all the other planets, orbited the Sun. It was such a fundamental change to our understanding of the Universe that it took over a hundred years to be accepted!

Comet ISON has been in a so-called "Retrograde loop" since November last year, moving westward in the sky. It reached it's Western most point on the 7th of April, and has now started heading back eastward.

Whilst we're proud of the images we're getting from the PWT, we can't quite match the fire-power of the Hubble Space Telescope. On April 10th, HST took a great image of comet ISON, which shows off the nucleus, coma and tail in much more detail than we can. To give a sense of scale, the coma in this image is a bit bigger than Australia...

Categories: Comet ISON | PWT