Star gazing with Astronomical Society president Toby Tobias

In mid-July, Saturn graces the north-western sky, with the blue-tinted variable-brightness star Spica, positioned seven O'clock down from it Below them is the brilliant white star Arcturus.

The easily recognisable constellation of Scorpio rides the sky, high in northwest, aloft of Saturn. The reddish-hued Antares, ‘the heart of Scorpio', is the most easily recognised star of that constellation.

The Southern Cross or Crux, the group of star featuring on our national flag, graces the southwest sky. Dotted around Crux are a number of fetching globular clusters, which are excellent objects to hunt with a telescope.

Astronomy uses three basic units of distance measurement: the astronomical unit (au); the light year (ly); and the parsec (pc). For example, 1 au is the average radial distance of the Earth orbiting the Sun, which equates to 1.49 x 10¹¹ m.

Generally, the unit is used for describing distances within our solar system. An example is 1 ly is the length light travels through a vacuum in one year; about 9.46 x 10¹⁵ m. Parsec is a contraction of the words parallax, method, and arc second. A parsec is the distance from the Sun to an astronomical object, which has a parallax angle of one arc second (¹⁄_3,600 of a degree). Essentially, you use trigonometry to calculate the distance of an object, using the angles from two measurements, taken six months apart when Earth is at its opposite ends, orbiting around the Sun. Because the angles are incredibly small they are normally in fractions of 1 second of arc: 1 pc equals to 30.9×10¹⁵ m. And, 1 ly is equivalent to 63,231 au and 0.3 pc respectively.

The Tauranga Astronomical Society observatory at its clubrooms in Fergusson Park, Matua. Public meetings are every second and fourth Tuesday of the month, at 7.30pm. For more information, phone 07 543 5358.

Data courtesy of Stardome, Auckland, and the Royal Astronomical Society of New Zealand.