Equinoctial planetary gathering

This week there is a celestial coming together involving Mars and Jupiter before dawn. Astronomers call a close approach between two or more astronomical objects an appulse.

The planets, which are in the constellation Sagittarius, appear closest together this Friday. Don't worry if you can't get up early though, because the planets will be within a degree and a-half of each other until Monday. If you want to know how large a degree is, look at your little finger at arm's length; the angle it subtends is, very roughly, one degree.

Jupiter is, by far, the brighter of the planetary pairing, shining as a bright yellow-white ‘‘star’’. In contrast, Mars is dimmer and gleams with a distinct orange/red hue.

If you have binoculars, or better still a telescope, you will be able to pick out the bright moons of Jupiter as brilliant ‘‘stars’’ near the giant planet. If you have a telescope and are looking at Jupiter at 3am on Friday, you will be able to see a dark ‘‘spot’’ crossing the planet's disc. This is the shadow of Io, one of Jupiter's moons.

While Mars and Jupiter seem close together in the sky, this is just a line-of-sight effect. At the time of the closest approach, Earth and Mars are 234 million km apart. Jupiter, whose distance will be 826 million km, is significantly further away.

While Mars and Jupiter are closest together in the early hours of Friday morning, this week's chart is drawn for tomorrow morning. The waning crescent moon will be located approximately halfway between Jupiter and Saturn, creating a remarkable early-morning vista.

The southern hemisphere autumnal equinox occurs at 4.49pm this Friday. This is the moment when the centre of the sun crosses the celestial equator heading north on its apparent annual journey around the sky. If the sky is clear, it's well worth looking out for auroras over the next couple of weeks. For reasons that are not fully understood, displays of the southern lights are statistically more common close to the equinox.

-By Ian Griffin

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