Saturday, August 15, 2009

What Can I See Tonight?

This summer I was at a star party, showing the sky to interested non-astronomers, as I do many nights each year. One participant came armed with the knowledge of a number of the finest objects to view in the sky. They asked me to show one, then another. They and the others there all enjoyed seeing each of these objects as I put them in the telescope's view.

Then they said "How about the Orion Nebula? I hear that's really beautiful!"

It is, but I couldn't show it to them. I had to apologize, then explain that we couldn't see Orion that night.


The Sun and the  Ecliptic, showing the Earth's orbit and position relative to the Sun.
The Sun blocks part of the sky from the Earth's view. Here, the area near Pisces is behind the Sun. Image by Tau'olunga.

As the Earth travels around the Sun during the year, there is part of the sky behind the Sun that is not visible. The part of the sky that is behind the Sun from our point of view changes depending upon where we are in our orbit. In the image above, Pisces lays on the far side of the Sun from the Earth. The other constellations nearby will also be hidden since they are only visible during the daytime. Pisces would be high in the sky at noon, but you can't see stars at that time because the atmosphere will be lit by the Sun.

Even if you were outside the Earth's atmosphere, there will still be stars hidden behind the Sun. But you would be able to see more of those close to the Sun since the Sun's glare wouldn't be spread out the way it is when you're looking from within the atmosphere.

Six months later, the Earth is on the other side of the Sun. At that time Pisces will be high in the sky at midnight. But the stars in Virgo and the area around it will now be hidden by the Sun.

M-42, the Orion Nebula.
The Orion Nebula. Impressive, but don't expect to see it in summer. Image by Filip Lolić.

Orion is called a "Winter" constellation because it is highest in the sky at midnight during winter. It's close enough to the ecliptic that it's hidden by the Sun during summer. That means the nebulas in it, including M-42, the Great Orion Nebula, will not be visible in summer. They are visible at other times of the year. Though only in winter are they visible at relatively convenient times of the night, near sunset. At other times you'll need to either get up early or stay up late.

There's a myth about Orion and Scorpius. It's said that they were mortal enemies and would always fight. In the sky this is reflected by the fact that they are at opposite sides of the sky, and are never in the sky at the same time. So, while Orion cannot be seen in summer, the wonderful sights of Scorpius are not visible in the Winter. So if Scorpius is up, Orion isn't, and vice versa.

The M-4 Globular Cluster
The M-4 Globular Star Cluster, in Scorpius. See it in the summer, you won't be able to in the winter. Image by Ole Nielsen

How much does the sky change each night? A reasonably accurate rule of thumb is that the stars rise three minutes earlier each night. Note that this doesn't apply to the Moon, it moves in an entirely different way, coming up about 40-some-odd minutes later each night. Another rule of thumb is that the sky changes by about 1 degree each night. This is because the Earth moves about 1 degree in its orbit about the Sun each day. This means that something that is up tonight by about 10 degrees above the western hemisphere will disappear in about a week and a half. So see it while you can!

Likewise, if you see the Pleiades just above the horizon at sunset, you have less than an hour to wait before the rest of Taurus, including Aldebaran (the Eye of the Bull) and the Hyades (another nice star cluster in Taurus) are visible.

Saturday, July 25, 2009


About a century ago, it was thought that everything that existed was within a few thousand light years of us. (See How Far? for an introduction to astronomical distances.) Our Sun was known to be one of many suns in the midst of a large structure made up of stars, collections of stars in the form of star clusters, and clouds of gas and dust called "nebulas" after the Latin word for cloud.

There was one class of nebula that was particularly interesting to many scientists. They were known as "spiral nebulas." One reason that these were of particular interest was that they were seen by some as new solar systems in the act of creation. Scientists such as LaPlace and Gould had developed ideas about how systems of planets like ours could come into existance. If the spiral nebulas were new planetary systems being formed, studying them would give the scientists more information to choose from the available theories and enough information to refine them or develop new theories.

M-51, the Great Spiral Nebula
M-51, known as The Great Spiral Nebula until about 80 years ago.
The shape of the "universe", as it was seen then, was pretty well known. The Milky Way, a band of light across the sky, had been recognized long before as being made up of stars. It was known that most of the stars lay in a more or less flat circular area, shaped like a plate. It was also recognised that there were more stars in the direction of Sagittarius, and that they extended over a larger area than the thin plate, there was a bulge. It was thought that this was where the center of the galaxy lay. This last wasn't apparent at first, since the stars at the center of the galaxy can't be observed directly from Earth. There are dark nebulas between us and the center of our galaxy, otherwise we would see a huge globe of light when we look that way at night.

In ancient times the Milky Way was thought to be a cloudy mass. When Galileo turned his telescope on the Milky Way about 400 years ago, he saw that it was individual stars in such great numbers and so faint that they blend into a cloudy mass when viewed without a telescope. The spiral nebulas did not look like stars, though. They looked like clouds, even through telescopes.

What Is It?

In the early part of the 20th century, some scientists felt that the spiral nebulas were not as close to us as other stars. They felt that they could not be new solar systems forming, because they were so far away that they would have to be many light years across, far larger than out solar system. Finally, it became possible to measure distances to the spiral nebulas. The distances were shocking to those who saw our universe as being a few thousand light years across. The distances to the spiral nebulas were in the millions of light years.

Further study determined that the spiral nebulas are made out of stars. They are so far away that they don't resolve into stars as easily as the other known star clusters that are only hundreds or thousands of light years from us. At first, these other bodies of stars so far from us were called "island universes." But this was a mis-use of the word "universe." The word universe implies everything. There can't be more than one "everything." If they were there, they were part of "everything", too.

Up until that time the word Galaxy had been the name for the Milky Way (which is what the word means.) It got generalized into the common noun galaxy as a word for these other groups of stars millions of light years away, and the word universe went back to meaning absolutely everything. Though today the word universe gets misused as a way of describing other sorts of places that you can get to in mystical or science fictional ways. The rule is, though, that if you can get there or interact with it in any way, it's still part of the same universe. The only way something can be another universe is if there's no possible way to have anything at all to do with it.

How Far Can You See?

One galaxy outside our own is commonly known as being visible to the eye. This is the Andromeda Galaxy. While all the other stars, planets, and cloudy patches you can see in the sky with your eyes are from a few light seconds (the Moon) to a few thousand light years away, the Andromeda Galaxy is about 3 million light years away. It's a fuzzy patch in the constellation of Andromeda. It doesn't look like much, in fact it's hard to see if you look directly at it unless you're in a really dark area with good skies for astronomy overhead. It shows up a little better if you look a little bit away from where it is because our eyes are more sensitive to slight differences of light a little off center of our field of vision.

An early photograph of the Andromeda Galaxy, then thought to be a nebula in our galaxy.
A photograph of the Andromeda Galaxy from 1888. At the time Andromeda was thought to be a nebula within our galaxy.
Occasionally other galaxies can be seen if your eyes are good and the sky conditions are really, really good. I've seen the Triangulum Galaxy by eye, which is a bit further away than the Andromeda Galaxy.

Both these galaxies are close to the Milky Way, relative to many other galaxies. They're both part of what's called the Local Group of galaxies. A small telescope can see many more, also quite close to us, but also many that are outside the Local Group.

How Many Galaxies Are There?

Look at all the stars you can see in the sky. Now look through a telescope. There are about 100 million stars in our galaxy, scattered like glowing sand across a black sheet of paper.

There are more galaxies that we have seen than there are stars in our galaxy, however. There are well over a billion galaxies. Everywhere that we look where our view of deep space is not blocked by something in our own galaxy, we see countless other galaxies.

Deep space image showing hundred of galaxies in one very small area of the sky.
Everything in this photo that doesn't have four little 'diffraction spikes' of light coming from it is a galaxy.
The picture above occupies an area of sky so small that the tip of your finger held at arm's length would cover it with plenty of room to spare. Galaxies are scattered across the sky like this in all directions.

What Are Galaxies Made Of?

There's nothing mystical about the other galaxies in our universe. They are made of the same stuff as our own galaxy and the same stuff we see here on the Earth. They have the same elements in them, and they are made up the same way. Just because they are far away doesn't mean that they have different rules of physics than us. We see the same physics there as we see here on Earth.

They are very interesting, though. Just as you have to go to special places on the Earth to see a tall waterfall, there are special places in the universe that are unusual and beautiful.

To see more interesting galaxies, and take part in doing some galaxy science yourself, visit the Galaxy Zoo. You can see lots of interesting and beautiful galaxies there, learn something about how they are classified, and help with classifying galaxies yourself. There are so many galaxies that there aren't enough astrophysicists to study each and every one. So members of the public have been called upon to help! Head on over, and give it a look.

Friday, July 3, 2009

Daytime Observing with Your Telesccope

You can get plenty of use out of a telescope for astronomy even if it's not dark out. First, it's important to not use your telescope to look at the Sun unless you've got the right equipment. That means a full-aperture solar filter for starters, removing any finder scopes from the telescope, and a good knowledge of what you're doing otherwise. Those little solar filters that screw onto the eyepiece are not safe. They can crack from the heat of the concentrated sunlight on them, and if you're eye's there when they do crack, you'll get the full force of the sunlight it lets through on your retina. Not good.

There are plenty of things to look at other than the Sun, however, that don't require any special preparation of your telescope. The Moon is often up in the daytime, and while it doesn't show its detail as sharply through a daylit sky as at night it's still very interesting to look at. Just as at night, the best area to look is along the line between the day and night parts of the Moon, what's called the "terminator".

Several planets can also be observed during the daytime. Mercury is always close to the Sun, and Venus never gets too far away. Finding them during the day is the problem since they don't stand out as well as they do at dusk or in the night. If you have a chance, it's good to get familiar with where they are when you can see them easily, before you go out to find them in the daytime. Use their location with respect to the Sun as a reference, since that'll be your biggest signpost in the sky during the daytime.

Then, when you go to find it during the day you can start out with the end of the telescope covered, as well as any finder scopes. Start at the Sun, then move away from it. Stop before you get to where you think you need to be for the planet, uncover, and then move toward the planet while moving away from the Sun. Never move back toward the Sun while you're looking through the scope. It's also a good idea to have a second person on hand to make sure you don't move the scope back toward the Sun while looking through the telescope.

If you've got a computer on your telescope, that can put you on the planet once you get it aligned. If you have an alignment procedure that will work with you levelling the scope, setting it to north, and using the Sun as a reference point (line up the scope with the Sun with the covers on it and the finder--use the shadow of your scope to line up on the Sun, and don't look through it!)

Venus can show very nice phases during the day, looking like a little Moon.

Jupiter and Saturn can be seen during the day as well, they look like ghostly versions of themselves during the day. Seeing them during the day is quite an experience.

You can also see the brightest stars during the daytime. Sirius, the brightest star (other than the Sun, of course), is the easiest to see. Arcturus, Fomalhaut, Betegeuse and Rigel can all be seen in daylight as well.

You'll want a clear sky, with as little moisture as possible. The deeper the blue at the highest point in the sky, the better your chances.

Give daytime observing a try, and see how many "night time objects" you can collect!