2-1 Viewing the Sky

Where is that star?

If you're trying to locate a position on the Earth's surface, you do so through latitude and longitude:

For latitude, we use degrees north and south of the Equator; that's a natural dividing line. The farthest north you can go is 90°N (to the North Pole), and the farthest south you can go is 90°S (to the South Pole).

For longitude, we have to make an artificial dividing line, because no such natural dividing line exists for this. Scientists settled on the Prime Meridian, a line of longitude running through a suburb of London, England, in 1884 as this starting point.

This system works fine, and you can locate any point on the Earth's surface with two numbers. This is what Global Positioning System (GPS) satellites can help with, and we've become very reliant on this system in recent years. Our school's front door has a latitude and longitude of 43.74751°N, 79.19741°W.

There is a star in the northern sky called Alioth. There's nothing particularly special about it, although you could probably see it on a clear night from the city. It's circled in the picture below.

Its name comes from the Arabic name for it, alyat al-hamal, which means "the sheep's fat tail." Hindu astronomers in India called it Angiras, which was one of the seven wise "rishis" in traditional literature. Chinese astronomers called it Běi Dǒu wu, the "Fifth Star of the Northern Dipper" -- which gives you a hint about its location in the sky.

If you're standing in Toronto, to find this star, point yourself towards the north, then turn about 40° to the east. (Since "northeast" is halfway between north and east, that would be 45°.)

Then, once you've turned your body in the proper direction, look up a little more than halfway up the sky. Directly above you, your zenith, would be 90°, but this is only about 50° above the horizon.

What you've done is located the star using the altitude-azimuth (alt-az) system.

When we put this into practice for Alioth, this is what we get:

Altitude is how far above the horizon you have to look, and can go from 0° (right at the horizon) to 90° (at zenith). It's a little bit like latitude.

Azimuth describes in what direction you need to point yourself, in terms of the four cardinal directions of north, south, east and west. In this system, 0° is north, 90° is east, 180° is south and 270° is west. It's a little bit like longitude.

Constellations

Alioth has a more generic name as well: Epsilon Ursae Majoris. You might be able to pick out the name of the constellation, or easily-recognizable set of stars in the sky, from this name. The constellation's name in Latin is Ursa Major, which translates to "Great Bear" in English.

(The "Epsilon" part of the name comes from the fifth letter of the Greek alphabet, epsilon, written as ε. Normally this would mean the star is the fifth-brightest in its constellation, but here, confusingly, it's the brightest.)

(Also, consider this... the phrase "Alioth, or Epsilon Ursae Majoris" includes all three major scientific languages used in the western world in the past two thousand years: Arabic, Greek and Latin.)

There are 88 constellations officially recognized by the International Astronomical Union (IAU). Ursa Major is an important constellation visible in the northern sky, but we have to play connect-the-dots to "see" it:

Does it look like a bear? Maybe, if you use your imagination. Let's try rotating it:

That's a little better. These images were generated using Stellarium, an excellent, free program you can install on any computer to tell you what the sky will look like from anywhere on Earth, at any time on any day. You can also use the web version within a browser.

One of the nice things Stellarium can do is to flesh-out the artwork a bit, if your imagination can't put together a bear.

Different cultures have connected these dots in different ways. Inuit people in northern Canada didn't see a bear, they saw a caribou:

In Belarus, their traditional folk stories told the tale of a man named Elias, who drove a horse-cart:

These are only a couple of cultures' interpretations of the constellations in the sky. There's nothing saying you couldn't make observations of the night sky, connect the dots to form entirely new pictures, and tell everyone you know about your "new" constellations. You'll probably have a bit of convincing to do, so good luck with that.

Asterisms

An asterism is a part of a constellation which is easily recognizable, but it isn't a full constellation on its own. Two very good examples are the Big Dipper and Orion's Belt.

The Big Dipper

Within Ursa Major, there is a group of stars that are (a.) all very bright, easily visible even within a city, and (b.) visible all year-round. (Some stars within the rest of Ursa Major are faint and can be hard to see, except under ideal conditions.)

In North America, we call this asterism The Big Dipper, because it looks like a ladle that you might use to scoop soup out of a large pot. In the UK and Ireland, this asterism is called The Plough, because of how it looks like an old type of plough that farmers would use in their fields, pulled by an ox.

Many other countries in Europe refer to this as some sort of wagon or cart, as noted above with Belarussian folk stories. Chinese astronomers saw it as either a ladle or a horse-drawn chariot, Vietnamese astronomers saw it as a ship's rudder, and in the Philippines it's associated with a water pot called a tabo.

Orion's Belt

In the southern sky during the winter, the constellation Orion can be seen easily. It contains two of the brightest stars in the sky, Betelgeuse and Rigel.

In Greek mythology, Orion was a hunter carrying a club in one hand and a shield in another. You can very easily spot three bright stars in a nearly straight line: these are called Orion's Belt.

You can use Orion, and the stars in the Belt, to find stars in other constellations.

The Celestial Sphere

If you go outside on a dark, moonless night, away from the lights of any city, the night sky above you can look like a dome, with countless stars stuck to that dome.

If we extend this idea around the whole Earth, we can call the entire night sky around our planet the celestial sphere.

The north celestial pole would be directly above your head if you were standing on Earth's north pole. If you were standing on the equator, the celestial equator would be directly above you. And, if you were standing on Earth's south pole, the south celestial pole would be directly above you.

The north celestial pole is very close to a fairly bright star in our sky, Polaris. (Its name gives its identity away.) This star is the brightest in the constellation Ursa Minor, the "Little Bear." Ursa Minor can also be called The Little Dipper although here it isn't really an asterism like the Big Dipper is; it's the whole constellation.

If you draw an imaginary line from the two stars at the end of the "bowl" of the Big Dipper and extend it out from the bowl, that line gets very close to Polaris. It's an important star because, as the Earth rotates, since Polaris is near the north celestial pole, its position in the sky does not change.

In the above animation, the positions of the stars and constellations near Polaris are shown every hour, from about 6:30 pm through 6:30 am, in early January. You can see how, as the Earth rotates, the position of almost every star changes... except Polaris.

You can simulate this movement by standing in the middle of a room, and looking at a point on the ceiling directly above you. Keep looking at this point, but turn your body around in a complete circle: the point stays in one place, but all other places on the ceiling appear to rotate. (The ceiling doesn't rotate, of course -- you do.)

Countless cultures have used Polaris, and the fact that its position doesn't change, for navigation and direction-finding. Sailors on the ocean used Polaris at night to determine the direction their boats were headed (although during the day, or in cloudy weather, they would have been lost). Ancient Egyptians knew Polaris was special, so pyramids were built aligned to it, and the other cardinal directions.

Making your own Star Map

The National Research Council of Canada has an excellent planisphere, or star map, that you can build for yourself. If you go their Activity Book #1's website, get the PDF file for the Planisphere on page 5, print out the two cut-out pages and follow the directions, you will have a star map that you can use for most latitudes where Canadians live.

As the directions suggest, this does not take Daylight Savings Time into account, so for those parts of the calendar in which our clocks are shifted an hour ahead, you will have to adjust for that.

Practice

The Basics

1. How many constellations are officially recognized by the International Astronomical Union?

2. What is the difference between a constellation and an asterism?

3. Why has Polaris been an important star for many cultures throughout history?

Extensions

1. Find the names of the three stars in Orion's belt. Two of them have names that start with "Al-". From what language were these names borrowed?

2. Think about two observers, one standing on Earth's north pole, and another standing on the equator. Where in each observer's sky would Polaris appear?

3. Is there an equivalent "south pole star" like Polaris is for the north celestial pole?