Hamilton Amateur Astronomers
Maintained by Grant Dixon
I have seen pictures of Halley's Comet when it appeared in 1910, and I was eagerly looking forward to seeing it when it returned in 1986. However, to my great disappointment, I could hardly see it at all, even in binoculars. Why was there such a difference in the two appearances? The nature and orbit of the comet doesn't change, does it?
Brian Chire
The comet's orbit is relatively constant (although Jupiter's gravity causes some minor changes) but the position of the Earth relative to the comet was different for the two appearances. In 1910 the Earth actually passed through the tail of the comet (and will again in 2062) but in 1986, Halley's comet was quite far from the Earth at its closest approach to the Sun (perihelion).
I have always wondered why the Milky Way is not horizontal to our horizon when we look at it in the sky. In fact, in the summer, it appears overhead, almost 90 degrees to our horizontal perspective. Is this because the plane of our solar system is sharply different to the plane of the main part of the stars in our Milky Way galaxy? Is our solar system plane really that different (90 degrees) to the main galactic plane? Or does it also have something to do with the tilt of the earth on its axis?
Brian Chire
It is a combination of factors. Not only is the plane of the solar system (the ecliptic) tilted with respect to the plane of the Milky Way (galactic equator) but the Earth's axis is also tilted with respect to the ecliptic. The north pole of the solar system (north ecliptic pole) lies about 23.5 deg away from polaris in the constellation Draco (18h 00m R.A.) and the North pole of our Galaxy (north galactic pole) lies about 27 deg away from polaris in the constellation Coma Berenices (12h 51m R.A.). By the way, it is our tilt of 23.5 degrees from the ecliptic that accounts for the seasons. Also, during the winter the night side of the earth is facing away from the centre of the galaxy and during the summer the opposite is true.
Using the software program "Earth Centred Universe" to measure the distance between the north ecliptic pole and the north galactic pole shows that they are approximately 60 degrees apart. If you look in a star atlas such as "Sky Atlas 2000.0" you can see that the Galactic Equator is tilted 60 degrees from the Ecliptic.
It is probably a lot easier to understand this out under the night sky with a few star charts handy. Join us a one of our scheduled observing sessions to learn more.
Hamilton Amateur Astronomers
Maintained by Grant Dixon
