Astronomy        Part 2

Telescopes usually have a field of view of about 1^o at their lowest magnification. This gives 180 x 360 directions plus the same again to cover parts between circles. Thus to cover the entire night sky there are 129,600 ways to point a telescope from a site with a clear view all around. In practice, with hills, trees, houses and other objects, it is likely to be rather less than this for most of us. But that still leaves a vast amount of sky to explore and discover!

To help, astronomers use a latitude and longitude system called declination and right ascension. With equatorial mounts, the mount is set for your latitude, and its axis pointed at the North Star. By making a time adjustment using sidereal time (looked up on the Internet), or by setting at a known star and comparing results, the setting dial for right ascension can be positioned, and locations of objects set to correctly position the telescope. Many amateur astronomers use star hopping, finding known bright stars with the finderscope, and then moving so many degrees etc in a particular direction, and moving from object to object, especially useful with Dobson type mounts! The other option is an electronic setting motor system, that once set up per viewing session can find parts of the sky for particular astronomical objects automatically.  

It is difficult to comprehend even interstellar distances, let alone intergalactic space, as the distances and volume of Space is so huge and vast, even beyond imagination!

Consider the number of hairs we have; or the number of leaves in our gardens, or on a few trees; or how many billions of atoms make up our bodies, but even these sorts of numbers are small in astronomical terms.

We’ll start with a known vista and gradually work our way upwards in scale. If I go from sitting in my garden listening to the birds, and climb a nearby hill; on a clear day, I can see maybe 25 miles. Let’s times that by 10 to 250 miles a typical distance between some big cities. Times that by 10 and we get a typical continent width at 2500 miles. Times 10 again gets us the distance to circumnavigate the globe 25,000 miles. Look at your car speedometer and see how many times you’ve driven the equivalent of around the globe. In my case about 4 times, but that’s only 4/10 of the distance to the Moon in all my driving in nearly 10 years. This 25,000 miles isn’t enough to contain all satellites we have put into orbit. Geostationary satellites that give us satellite TV are ½ as far out again! OK times 10 again and we have reached the Moon at 250,000 miles. That’s 10,000 TIMES as far as I can see from my hilltop.

For the fifth step times 10, we get to 2.5 million miles which may contain some asteroids, but we have to go a further 10 times step, the sixth to reach the closest interplanetary distance for us of 25 million miles, when Earth and Venus are closest, ie 100 times further than between Earth and our Moon. Mars scarcely approaches closer than 34 million miles and that only every few years. So seventh step 10 times further again gets us to 250 million miles or round about 3 times Earth Sun distance. That comfortably contains Mars orbit around the Sun and some of the asteroid belt, but only gets us about halfway to the closest distance between the Earth and Jupiter. Times 10 again to 2500 million miles doesn’t get us out past Neptune. Remember our Earth Sun distance is a mere 93 million miles. So 9^th step times 10 to 25,000 million miles and we start motoring! This is enough to contain all of the planets nearly 2 ½ times over and contain some of the many comets that exist on the periphery of the solar system. But even a tenth step times 10 (ie ten thousand million times my hilltop vista) or 250,000 million miles, only gets us a short way into interstellar space.  Yet we already talking about a volume over 200,000 times bigger than the entire solar system.

We have to go up by 100 times steps from here to even start getting anywhere else from our own star system. We have to get to 25,000,000,000,000 miles to the nearest star, so that’s 100 times step 10 from the sphere containing all the comets of the solar system or in the plainest English, 100,000,000 TIMES further than any human has ever travelled, yes, 100 million times further than the Earth Moon distance to just the NEAREST star and back.

We’ll take a brief interlude here from our interstellar travels, and consider how we know that these distances are correct and not exaggerated. Even at this vast distance, the parallax effect across the Earth’s orbital width (nearly 200 million miles) is measurable, noting the stars very slightly different relative direction in the sky for stars ‘very close’ to us, measured at 6 month intervals. From there it is simple math to calculate the distance.  By comparing the relative brightness viewed from Earth for different star types (see part 1), their actual distance can be approximated, as with some objects such as Supernova in vastly distant galaxies, by comparison with known types at known distances.

Ok, just so the distance to the nearest star seems less daunting, it’s about 5,000 times the width of the Solar System up to Neptune away. That said, Neptune orbits at a colossal distance from the Sun!  From Neptune the Sun has a disk 300 times smaller than we see, just barely bigger (though much brighter) than a star!    

Now we take a simple 5x bigger step to get to a cube 20 light years across so that from here we can travel courtesy of Terence Dickinson- (do read his book ‘Night Watch’!!!!)  across the universe.

This cube contains just 12 star systems (8 single stars, 3 double stars 1 triple star) including of course our own Sun and us!

So 100x bigger again, 2,000 light years across and maybe 2 million stars.  But we’ve only got part of one of the spiral arms of the Milky Way! But still hundreds of times more stars than are visible to the naked eye. Most stars are hundreds of thousands of AU apart, though in some globular clusters may be up to 10 times closer eg around ½ light year apart.

100x again to 200,000 light years and we have just our Galaxy and its companion small galaxies. 100x yet again at 20 million light years across gets us 6 larger spiral galaxies and dozens of smaller galaxies making up the ‘local’ group of galaxies, of course including the Andromeda galaxy our ‘near’ neighbour.

But we have travelled 1 million times further than our local star group or 500 million million times further than our Earth Moon human travel limit.

There are mostly millions of light years between galaxies, though some do ‘collide’ and merge. Our own galaxy and the Andromeda galaxy are heading towards each other to meet in perhaps a billion years time.

There may be hundreds of millions of light years between galaxy groups.

Another 100x takes us to a cube 2 billion light years across containing numerous groups of galaxies; some in clusters, some in ‘ribbons’ and also many isolated individual galaxies. Perhaps in the region of 100 million galaxies in total.

We can only go perhaps another 12x to reach the edge of visible galaxies, even with the Hubble Space Telescope. From 14 billion light years to 15 billion light years is just a microwave haze thought to originate from the ‘Big Bang’ Creation of the Universe.

This cube up to the ‘edge of the Universe’ contains maybe 100 billion galaxies. Its width is about 1 million million million times our Earth Moon distance.

This is quite incomprehensible in human terms even of the volume of Space just between stars let alone galaxies. The Universe is far far vaster than astronomers’ prior to the late 20^th Century ever ever imagined or thought possible!

It is really humbling, just through a modest sized amateur telescope, to see (to even be able to see it at all!) a galaxy maybe 70 - 100 million light years away where light set out the equivalent of 1 million human lifetimes ago and to know that light setting out today from our own galaxy will take another million lifetimes, or so, to reach this other galaxy.

Or that from galaxies photographically visible in the Hubble Space Telescope light set out to us almost from the Dawn of Creation!!!!