Astronomy Basics
Why do we have night and day?
Day and night is caused by the Earth spinning on its axis (like a basketballer spinning a basketball on her finger). Only the half of the Earth facing the sun receives light from the sun. The other half is blocked from the sun by the rest of the Earth.
Day and night is caused by the Earth spinning on its axis (like a basketballer spinning a basketball on her finger). Only the half of the Earth facing the sun receives light from the sun. The other half is blocked from the sun by the rest of the Earth.
Why does the sun rise in the East and set in the West?
The answer is all to do with the way we are spinning. Spin towards your right arm, and notice objects will first come into view from the right and leave your view from the left. The earth spins from west to east... because it is spinning to the east, objects will first appear in the east and leave our view in the west.
So all objects that go underneath the horizon will rise in the eastern side of the sky and set in the western side of the sky.. the sun, moon, planets, stars all do it!
Below is a Scratch Animation of the Earth spinning on its axis, as seen from a spaceship below the Earth looking up at the South Pole (left). Note the Earth spins from west to East... western Australia is always spinning towards Melbourne on the east. The right hand side of the animation shows the same scenario from the point of view of the man. If you try and tilt your head to always be in line with him, you'd get the same image as you get on the right... (notice it is the same image as the one on the left, only tilted around to make the man on the top.
Notice the rising in the east and setting in the west is simply due to the direction in which the Earth spins.
Push 'r' and SPACE at the same time to reset it.
Why can't we see the stars during the day?
The reason we can't see stars during the day is because the sun's blue light is scattered by our atmosphere across the entire sky (as we saw in the Light Unit). All this blue light everywhere is just bright enough to drown out the faint light of the stars. So we lose the stars in the flooding of blue light.
At night, light from the sun doesn't reach us because the other side of the Earth blocks it. With no blue light scattered across the sky, the sky becomes its true transparent self, and the starlight from the stars is able to shine through it and into our eyes.
Only on planets with atmospheres that scatter sunlight across the entire sky do we find the stars not visible during the day. On the moon, with no atmosphere, you can see the stars even in the middle of the day, and the sun remains just a bright light in the sky...its light isn't scattered. Below shows the middle of the day on the Moon. Note you can see the saucepan (upside down) in the constellation Orion near the horizon below the sun.
The reason we can't see stars during the day is because the sun's blue light is scattered by our atmosphere across the entire sky (as we saw in the Light Unit). All this blue light everywhere is just bright enough to drown out the faint light of the stars. So we lose the stars in the flooding of blue light.
At night, light from the sun doesn't reach us because the other side of the Earth blocks it. With no blue light scattered across the sky, the sky becomes its true transparent self, and the starlight from the stars is able to shine through it and into our eyes.
Only on planets with atmospheres that scatter sunlight across the entire sky do we find the stars not visible during the day. On the moon, with no atmosphere, you can see the stars even in the middle of the day, and the sun remains just a bright light in the sky...its light isn't scattered. Below shows the middle of the day on the Moon. Note you can see the saucepan (upside down) in the constellation Orion near the horizon below the sun.
Why do we see different stars at different times at night
The stars you see in the early evening just after the sun has gone down are different to the stars you see in the morning just before the sun comes up. Why?
Again, it is all to do with the spinning Earth. The stars we see depends on which direction Melbourne is facing on the spinning Earth. Between 6pm and 6am, the Earth rotates around 180 degrees, so Melbourne is directed at different stars in the evening than in the pre-sunrise morning.
Below is a Scratch Animation showing the spinning Earth. In this simplified model, we have made the rest of the universe completely still. The bit down the bottom represents the view of the sky as seen by the person on Earth with the centre of this bottom bit representing the direction he is facing (red arrow in top part). It's only rough, but it should give you an idea of why the sky seems to change over the space of a night and day.
If the sun's blue scattered light didn't flood the sky during the day, we'd see all the different constellations over 24 hours. As it is, we only get around 12 hours of seeing stars each day.
Why was I able to see the saucepan in January but then I couldn't see it in June?
Why was I able to see the saucepan in January but then I couldn't see it in June?
At midnight, Melbourne is directly opposite the sun (very close anyway). The stars we see at midnight depends on where the Earth is relative to its orbit around the sun. Thus, in January, Orion and the saucepan stars are in the other direction than the sun and so we see it at midnight. In June, because the Earth has now moved to the other side of its orbit, the Orion and saucepan stars are in the same direction as the sun. Thus, at midnight, the Earth is blocking the view to the saucepan stars. During the day, the saucepan is there in the sky, only the sun's scattered light is flooding the sky and stopping us from seeing the stars. In the animation below, we see the Earth orbit around the sun. The arrow always points to a person at midnight and what they see. Notice the stars at midnight keep changing.
What is the Celestial Sphere?
The Celestial sphere is an imaginary sphere in space that surrounds the entire Earth with the Earth in the centre.
The Celestial sphere is an imaginary sphere in space that surrounds the entire Earth with the Earth in the centre.
By adding this sphere to our view from Earth we find that each star/planet/moon gets its own unique place on the sphere. Just as you can divide the Earth's surface up into sections using lines of longitude and latitude, you can use lines to make sections on the celestial globe. Each star will belong to a particular section (see below, notice when we put the spherical grid on, the sun belongs to the section 2nd from the left, and 2 sections up from the middle horizontal line.
We call the point on the celestial sphere directly above the Earth's North Pole, the North Celestial Pole.
We call the point on the celestial sphere directly below the Earth's South Pole, the South Celestial Pole.
We call all the points on the celestial sphere that are lined up with the Earth's equator, the Celestial Equator.
Just as all other points on the Earth are given their own longitude and latitude, each point on the celestial sphere has a declination (latitude) and right ascension (longitude). The '0' line of longitude on Earth is a line connecting the North and South Pole and going through London (called the Prime Meridian). The '0' line of right ascension (longitude) on the Celestial Sphere is a line connecting North and South and going through a point called the vernal equinox.
We call the point on the celestial sphere directly below the Earth's South Pole, the South Celestial Pole.
We call all the points on the celestial sphere that are lined up with the Earth's equator, the Celestial Equator.
Just as all other points on the Earth are given their own longitude and latitude, each point on the celestial sphere has a declination (latitude) and right ascension (longitude). The '0' line of longitude on Earth is a line connecting the North and South Pole and going through London (called the Prime Meridian). The '0' line of right ascension (longitude) on the Celestial Sphere is a line connecting North and South and going through a point called the vernal equinox.
Each star has its own unique location (a latitude and longitude basically) on the celestial sphere which represents the direction of that object from Earth.
If every star moved in a straight line towards the Earth until they were equal distance from the Earth, and that distance corresponded to the radius of our celestial sphere, then each star would actually be touching the surface of the celestial sphere.
If every star moved in a straight line towards the Earth until they were equal distance from the Earth, and that distance corresponded to the radius of our celestial sphere, then each star would actually be touching the surface of the celestial sphere.
Since the dawn of humans, we have made patterns with the positions of stars in the sky, relative to each other. Each pattern is called a constellation. Some constellations are shown on the celestial sphere below.
How many constellations are there?
The entire celestial sphere (representing the sky in all directions including underneath you) is divided up into 88 constellations.
Boundaries between each constellation are then made, thus dividing the whole celestial sphere into 88 regions (like countries).
These 88 regions cover the whole sphere with no overlapping and no gaps. Thus, every faint star and galaxy belongs to a particular region, named after the constellation found in that region.
The 12 most famous constellations are the ZODIAC constellations.
What are the Zodiac Constellations?
From Earth, the sun's light blocks out the sky during the day, not allowing us to see the stars and constellations that are there. But get out of the atmosphere in a spaceship that follows the Earth around its orbit, and you get this:
The 12 most famous constellations are the ZODIAC constellations.
What are the Zodiac Constellations?
From Earth, the sun's light blocks out the sky during the day, not allowing us to see the stars and constellations that are there. But get out of the atmosphere in a spaceship that follows the Earth around its orbit, and you get this:
As the Earth moves on its orbit, the position of the sun relative to the background stars and the celestial sphere keeps changing. However, in one full year, the sun appears to move across only 12 of the 88 constellations. These 12 constellations are known as the Zodiac Constellations.
Sagittarius, Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius.
Your 'star sign' is based on the position of the sun when you are born. If you are born when the sun's image on the celestial sphere is on Gemini, then you are a Gemini* (*except for the fact that the original dates were made for 1000 BC and are OUT now. To get your real star sign, pretend you were born 1 month earlier and work out what star sign that date has. That will be your real star sign.
Sagittarius, Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius.
Your 'star sign' is based on the position of the sun when you are born. If you are born when the sun's image on the celestial sphere is on Gemini, then you are a Gemini* (*except for the fact that the original dates were made for 1000 BC and are OUT now. To get your real star sign, pretend you were born 1 month earlier and work out what star sign that date has. That will be your real star sign.
Why have I never seen famous constellations like the Big Dipper?
The stars of the Big Dipper are in such a place in the universe that their positions on the celestial sphere are near the north celestial pole.
The stars of the Big Dipper are in such a place in the universe that their positions on the celestial sphere are near the north celestial pole.
Due to our position in Melbourne, when we look up at the sky, we are facing down somewhat.
As the Earth rotates, there are some parts of the celestial sphere which are ALWAYS blocked by the top part of the Earth from our view. The Big Dipper fits in this area. So this is why we never see the Big Dipper (see the Big Dipper's position on the picture 2 above). Also, note from the picture 2 above, that the southern cross is in the southern part of the celestial sphere. From Melbourne, it is nearly always visible. But for people in North America, they can't see the southern cross because the lower part of the Earth blocks it from their view.
NOTE: The Big Dipper is NOT a constellation but a asterism (a popular pattern that isn't an official constellation). The Big Dipper actually forms part of a constellation known as the Big Bear. Also note, some people think the 'saucepan' is the same as the big dipper. It is not. The saucepan, which we can see in Summer and Autumn, is part of the constellation Orion the Hunter.
NOTE: The Big Dipper is NOT a constellation but a asterism (a popular pattern that isn't an official constellation). The Big Dipper actually forms part of a constellation known as the Big Bear. Also note, some people think the 'saucepan' is the same as the big dipper. It is not. The saucepan, which we can see in Summer and Autumn, is part of the constellation Orion the Hunter.
What constellations should I be able to see in the night sky?
You should be able to see the Southern Cross (real name Crux=latin for Cross) at all times of the year. There is also the False Cross which is slightly bigger. To ensure you are looking at the real Southern Cross, ensure there are 2 pointers pointing towards it. The Southern Cross will either be above you, or somewhere in the sky when facing South.
You should be able to see the Southern Cross (real name Crux=latin for Cross) at all times of the year. There is also the False Cross which is slightly bigger. To ensure you are looking at the real Southern Cross, ensure there are 2 pointers pointing towards it. The Southern Cross will either be above you, or somewhere in the sky when facing South.
You should be able to see Orion the Hunter which includes the saucepan. In Summer it is roughly overhead when the sun goes down. By Autumn, it will be in the west and set a few hours after the sun goes down.
The other constellation to see is Canis Major (Big Dog) which includes Sirius, the brightest star in the night sky (planets Venus and Jupiter are brighter but are not stars). Canis Major always comes after Orion, as in the old legends, it was a dog following his master, Orion, the hunter. For the picture on the right below, Sirius is the brightest star. Below left are 3 stars (the tail and back leg of the dog) which form an arrow pointing somewhat towards Sirius.
Do we have to keep changing the positions of stars on the celestial sphere each year as the stars move?
While all visible stars are moving through the galaxy in their own unique way... the stars are so far away from us that their positions in the sky don't change noticeably at all. So a map of the sky/celestial sphere made 100 years ago will still be very accurate today.
To get an idea of this, consider a boat race.
All boats are moving the same distance at the same speed. Yet from your vantage point, the boat at the front's movement is more noticeable than one further back. By the time you get to the one at the back, its movement isn't noticeable at all. This is what it is like with the stars.
The celestial sphere's star positions works similar to a globe of the Earth. Technically, the continents are moving and in 100 million years the Earth will look very different. But over our lifetimes, the globe of the Earth is accurate. 12 cm change per year isn't noticed on a globe of the Earth.
Similarly, the stars positions on the celestial sphere will change over millions of years. But in our lifetime we can pretend they are constant.