Trigonometric parallax is an easy method to calculate the distance from earth to the planets and stars in what we call: how to measure starlight. By measuring the angle of the apparent movement of an object against the background sky from either end of a known baseline, such as the diameter of the earth allows the distance to the object to be worked out using simple mathematics. The larger the baseline, the greater the distance that can be calculated. For nearby planets earth`s diameter is a long enough baseline. For nearby stars astronomers use the diameter of the earth`s orbit as they observe the annual shifts of a star.

Until recently, the range of the parallax method was only about 100 light years. But the range has now been extended with the help of the satellite `Hippapchos` launched in 1989, which can measure the position of stars far more accurately than ground based telescopes, whose readings are made less precise by the earth`s unsteady atmosphere.

The accuracy of `Hipparchos` is two thousandths of an arc second, 30,000 times more accute than the naked eye. The satellite has been used to provide the distance of stars out around 1,000 light years.

One of the farthest stars than can have its distance measured using the unique parallax method, is Rigel (Beta Orionis), which is about 900 light years away. Closer to us, at 8.6 light years is the brightest star in the night sky, Sirius (Alpha Canis Majoris). The cloest star to our earth is Proxima Centuri, which at 4.3 light years away, has an annual parallax angle of just under 1 arc second (three ten thousandths of a degree.)

But whereas we can use parallax method to measure the distance of starlight, light from nearby objects reaches the human eye so fast that it appears as though it`s now. But science suggests what we see in the universe happened some time ago!

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