Astronomy question: Distance 0.254 AU? What's that mean?

Question

I'm looking at "Spaceweather.com" (cool site, you should check it out). They're showing the orbit of comet 8P/Tuttle, and defining the distance on January 2, 2008 as being 0.254 AU. The problem for me is that I don't know what that means in terms of distance.

Wickapedia isn't helpful to me, and I'm not having much luck finding a site that will define astrological terminology, things like Right Assention. Some of the terms I can understand, like Declination (correct me if I'm wrong) which means the angle above the horizon. I'm sure there are more calculations to take into account, but I'm interested in learning.

I have a Dobsonian 4 1/2 inch reflector and a Meade 3 1/2 refractor and some cheap (not the expensive ones) eye pieces. I enjoy looking at things like Jupiter, Saturn, Mars, Venus, and (rare) Mercury (of which I've seen through my refractor once).

When time permits, I may go back to schooll to learn more on astronomy, but for now you can help.

Thanks.

Answer 1

Aero and Howard have slipped a few decimal places - 1 AU is about 93,000,000 (93 million) miles. Declination and Right Ascension (RA) are the two coordinates that locate objects in the sky. Declination is the angle above the equator; RA is the angle from the point where the sun is at the vernal equinox, expressed in time units (24 hrs = 360 degrees). Angle above the horizon is called simply altitude, wile the angle around the horizon is called azimuth (and is measured in degrees).

Answer 2

1 AU is one astronomical unit, or the average distance from the sun to the earth - about 93,000 miles. So .254 AU would be roughly 23,000 miles.

ADDED: Right Ascension and Declination are the numbers used to specify locations of objects in the sky using an equatorial mount. Since you have a Dobsonian with an alt/azimuth system, you do not need these numbers, although you should probably gain a clear understanding of what they mean - they represent a spherical coordinate system based on angle from the polar axis (declination) and meridian angle from an arbitrary point in Pisces (right ascension).

ADDED (2) : Woops - Good catch Injanier - sorry for the bum steer!

Answer 3

AU stands for Astronomical Unit which is the average distance from the earth to the sun, approximately 93,000 miles.

Answer 4

One AU is defined as the average distance between the earth and the sun. It is about (I think) 1.5*10^8 km.

Answer 5

1 AU (Astronomical Unit) is the distance from the Earth to the Sun.

Answer 6

Thanks for pointing that out INJANIER, I was scratching my head.

In terms of distances, check out RYAN question about asteroid 2007TU24, five questions down from yours.