Question 20S 12: Our Place in the Universe,Questions, Short Answer, Starting Out, galaxy, planetary orbit, observing the universeMeasuring distances within the solar system and beyond


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Question 20S - 12: Our Place in the Universe,Questions, Short Answer, Starting Out, galaxy, planetary orbit, observing the universeMeasuring distances within the solar system and beyond
Astronomers use different distance units as the scale of the distance requires, going from metres to astronomical units (linking with parallax measurement) to parsecs and finally to light-years.
Distances in metres

From the Sun to the Earth is 149.6 million kilometres, or 149.6 thousand million metres. Pluto is 5907 million kilometres from the Sun, a number well beyond the scale of ordinary human experience.
The astronomical unit

The mean radius of the Earth’s orbit is called an astronomical unit or AU.

1. If Jupiter is 778.3 million kilometres from the Sun, how many times further away than the Earth is it? How far is this in AU?
2. Estimate the distance from the Sun to Pluto in AU



The parsec

A unit of distance in common use amongst astronomers is the parsec. As the Earth moves in its orbit around the Sun, the position of nearby stars against the background of very distant stars seems to change.


The parallax of a star is the angle labelled a on the diagram. These angles are very small because even nearby stars are very distant compared with the Earth’s orbital diameter. Small angles can be expressed in minutes and seconds; one minute of arc is 1 / 60 of a degree; one second of arc is 1 / 60 of a minute. If the radius of the Earth’s orbit is R then the distance to a star with parallax a is:

A star at a distance of 1 parsec (1 pc) from us has a stellar parallax of 1 second.


3. How far is 1 parsec in metres?

4. How far is this in astronomical units?


The light-year

The nearest star to us is Proxima Centauri and light takes 4.2 years to reach us from it. On the Earth, we are used to light apparently travelling instantaneously, but even at the ultimate speed in the Universe (approximately 3.0 ´ 108 m s–1) light still takes 3.3 microseconds to travel a kilometre. Distances even in the solar system are vast and the radio signals that travelled from the first men on the Moon to Earth took long enough to cause considerable delay to communications.

5. The Moon is 3.84 ´ 105 km from Earth. What would the minimum time delay be between a question asked on Earth and its reply arriving from the Moon?


6. How long does light take to reach us from the Sun?

7. How far does light travel in one year? How far is Proxima Centauri from us in metres? In AU? In parsecs?


These last two answers should have convinced you that using light travel time is the only sensible distance unit to use for the stars and beyond. Our Milky Way galaxy is 100 000 light-years in diameter; the nearest galaxy to ours is the great Andromeda galaxy, M31, a distance of 2.2 million light-years from us. In fact, this is said to be the most distant object visible to the unaided eye – on a very clear night.

8. How far is it to Andromeda in AU? In parsecs?


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