The sun is about to move over the northern hemisphere and we won't get it back for another six months. This evening in Australia will mark the autumnal equinox, when the sun passes directly over the equator on its way north. This passage occurs just before 9:30pm Sydney time and close to 6:30pm in Perth. Australia will soon be on the downhill run into winter as days become shorter than nights. This starts to happen a few days after the autumnal equinox due to effects relating to the size and shape of the sun and the refraction of light in our atmosphere. The sun will venture back over the equator on 23rd September, although Australia has a whole winter to get through before then.