1-1 Objects in our Sky

Day vs. Night

This sounds like a silly question, but it's actually not quite so obvious:

Why is the sky dark at night?

The Earth rotates on its axis, once every day. If you're on a part of the Earth that is facing the Sun, it's daytime. If you're on a part of the Earth that is facing away from the Sun, it's nighttime.

During the day, the light from the Sun makes our sky bright. The reason our sky is blue during the day is due to something called Rayleigh scattering, and this video clip nicely explains things.

The Sun appears to "set" below the western horizon, but this is only because the Earth is turning. If you're looking towards the Sun, you're going to eventually move to the nighttime side of the Earth, as shown in this animation.

When our sky isn't being lit up by the light from the Sun, we can see the stars and other planets. Occasionally we can see something more exotic like a comet or a meteor (a "shooting star"), but we'll get to those later.

Things that move in the sky

As long as there have been humans, we have been looking at the night sky, and the points of light that dot the blackness.

Most of those points of light in the sky stay "fixed," relative to one another: we call these stars. They form patterns that are called constellations, where we play connect-the-dots and say things like, "That group of stars looks like a bear," or "That group of stars looks like a hunter." Sometimes this takes some imagination, though!

The above group of stars is the constellation Cygnus. On the left is a connect-the-dots version... does it look like a swan flying downwards? The right version fills in the artwork a bit more, which helps. We'll look at stars and constellations later, in more detail.

If we take away those fixed stars, there are seven things we can see, without a telescope, in the sky that move:

1. the Sun

2. the Moon

3. Mercury

4. Venus

5. Mars

6. Jupiter

7. Saturn

The number seven appears in a lot of different religious and spiritual systems as a special number. Is this why? Definitely some food for thought.

While the Sun and Moon are relatively large, the five other objects have a few other properties in common:

• they are small points of light

• sometimes they look like very bright stars, other times they look like faint stars

• you can't watch them move hour-to-hour against the fixed stars, but if you track their position through weeks and months, they move

It might not seem that important or exciting to you to track the movement of a point of light against other points of light, but perhaps our ancestors didn't have much else to do at night other than look upwards.

Over the millennia, these five points of light that moved were given various names by various groups of people. Collectively, we call these slowly-moving points of light planets, which comes from a Greek word meaning "wanderer."

Don't be fooled by the rotating Earth

Let's say you're standing outside, looking up at the night sky. Do you feel yourself moving? Probably not.

If you were to come back to the same spot an hour later and look up, your view will have changed a bit: the same stars which were right in front of you have moved off a bit to the west. We can simulate that view using Stellarium; here, the observer is looking towards the southwest, from hour to hour.

Since you don't feel yourself moving, you might think that the things in the sky, also called celestial bodies, move. But, rest assured, you are still on a rotating Earth, which gives the illusion that the sky is moving around you.

It's very convincing, and in recent years some public figures have said very silly-sounding statements that suggest the Earth is, in fact, flat. Don't be fooled, though: the Earth is round, it rotates on its axis, and we go around the Sun, just like every other planet does.

Indigenous views of the sky

On Turtle Island (the name some Indigenous groups give to North America), many different peoples have been observing our night sky for tens of thousands of years. They, along with other groups from all around the world, have observed the stars, the planets, the Moon and the Sun.

The different types of stories, folklore and culture that have been built by Indigenous groups to explain the things in the sky, and their connections to those things, are endlessly rich and intricate. It's important to learn about these different ways of knowing about the natural world.

The University of Calgary has an excellent website that describes what a lot of western Canadian Indigenous groups have developed about the night sky.

For example, this story about the planet Venus, the third-brightest object (and the brightest planet) in our sky, comes from the Siksika Nation in southern Alberta. (Venus is often called the "Morning Star" because it can be seen in the morning just before the sunrise. It's called the "Evening Star" when it's visible just after sunset. But, of course, we now know it's not a star: it's a planet.)

In Australia, the Indigenous peoples there have been observing the sky for roughly the last 65 000 years -- possibly the oldest continuous cultures, and thus astronomers, on Earth. This website is a great resource which explores different groups' stories. One such set of stories concerns the Sun and the Moon, which you can read here.

Practice

The Basics

1. How many things appear to move in our sky?

2. What is true about the Earth and the Sun if you are experiencing (a.) daylight, or (b.) nighttime?

3. Explain how "sunrise" works.

Extensions

1. Do a little research to determine the difference between the "rotation" and "revolution" of the Earth.

2. Does your culture have stories about stars, the Sun and Moon, and the planets? If you're not sure, ask someone else in your culture or do some online research.