Starwatch: moon marks the equinox with cruise past Antares

How proximity to the horizon affects the colours of the moon at different points on Earth

Celebrate the equinox this week with the waxing crescent moon, low in the south-south-west, cruising past the red star Antares in Scorpius, the scorpion.

The chart shows the view from London at 20:00BST on 21 September. The moon will be approaching its first quarter (half-moon) phase with around 39% of its visible surface illuminated. When it is this low against the horizon, its usually silvery glow will likely be transformed into a ruddier colour. This is because the blue component of its light is scattered out of our direct view by the molecules in the atmosphere. When the moon is close to the horizon, we must look through more of the atmosphere than when it is high, near the zenith, and so the effect of losing the blue light is more pronounced.