So what’s a supermoon? You may have heard about the supermoon that will appear on November 14 2016. Wiki claims the definition of a supermoon is “..a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit.” The idea began in the field of astrology, not the science of astronomy, and the definition is completely arbitrary. Even NASA promotes them. And now they’ve upped the ante by defining an “extra-supermoon” that will be visible on the night of 14-15 November 2016.
When, exactly, does it happen? On November 14 the Moon rises at 7:07pm AEDT (that’s at Sydney, it’s a little earlier or later if you live elsewhere). It reaches its closest point to Earth (or perigee) in this orbit at 10:21pm AEDT when it is just 356,509km away. The Moon finally reaches its full phase at 00:52am AEDT on November 15 when it will have moved out a little to a distance of 356,520km.
So this perigee is a bit special, it being the closest this year. It is also the closest perigee since January 26 1948 (356,461km), and the closest until November 25 2034 (356,445.4km). And the full Moons? Well, the full Moon on November 14 is the closest this year. And its the closest full Moon since January 26 1948 (356,490km), and the closest until November 25 2034 (356,445.7km).
So much for the numbers! What will I see? When the Moon rises on the 14th it will be indistinguishable from any other full Moon. But it will look big – because it always looks big near the horizon. This is the well-known moon-illusion effect, the cause of which is still debated (I find the Ponzo illusion explanation convincing). By the time the Moon reaches perigee three hours later it will be higher in the sky and will actually appear smaller – again this is due to the moon-illusion. When it finally reaches full phase, after midnight, the Moon will be high in the sky and will look just like any other full Moon you’ve ever seen.
The situation is illustrated in the figure above. A standard biscuit (Arnott’s Kingston) is shown rising in the east beside a superbiscuit (Arnott’s Delta Cream*). Both biscuits appear larger than expected due to their proximity to the horizon. The difference in their apparent size has been exaggerated and is also more apparent when seen side by side on the same day.
Is it worth watching? A full Moon rise is always a super sight whether or not you can detect an increase in its size. So find a hill, headland or beach with a good view to the east and enjoy the November 2016 “extra-supermoon” rise.
*No endorsement implied, they just happened to be nearby. Clearly Arnott’s works in metric units, the Delta Cream is precisely 50mm in diameter, not 50.8mm.