Camden Howitt, of Thorrington School, asks :-

How do scientists measure the huge distances in space?

Alan Gilmore, an Observer-Technician at the University of Canterbury's Mt John Observatory, responded.

These days the distance to the sun and planets can be measured by timing how long a radar echo takes to return to us.

Before they had radar, astronomers used 'parallax' to measure planetary distances. A nearby planet appears against different background stars when viewed from different places on earth. We call this effect parallax. Knowing the parallax angle and the distances between the places we can calculate the distance of the planet. We use our two eyes in the same way to judge short distances.

There is a strict rule about the distance of a planet from the sun and the time it takes to circle the sun. So once we know the distance of any planet from earth we can calculate the distances of all the other planets.

To measure the distances to stars, astronomers use the parallax caused by the earth orbiting the sun. A star is photographed when the earth is on one side of its orbit then again six months later when the earth is on the other side. A nearby star shows a very tiny shift against distant stars.

Once we know how far away a star is we can calculate its true brightness. Knowing the true brightness of stars gives us the first of a series of 'yardsticks' that finally allow estimates of distances to distant galaxies.