January 16, 2018

Astro-Event: A Very Old Moon Pairs with Venus.

This week’s astronomy challenge ties in two potential visual challenges: sighting a very slender crescent Moon and a daylight occultation of Venus. A grouping of the next two brightest objects in the sky after the Sun is always a treat; the challenge comes from the fact that the celestial pairing will be very close to a brightening dawn horizon. Venus just passed inferior conjunction on October 28th; it will sport a 2% illuminated crescent about 60” seconds in size at magnitude -4.2. The Moon, meanwhile, will be about 1% illuminated and reaches New on November 6th at 04:52 Universal Time.

At about 24 hours from New, both will be tough but not impossible to spot; start sweeping the horizon about 30 minutes prior to sunrise for best results. Lucky observers positioned from southern Africa to Western Australia will have an additional challenge; a daylight occultation of Venus by the Moon on November 5 centered on 07:00 Universal Time. The best odds at catching this successfully would be to lock a polar aligned scope on Venus before sunrise and track it throughout the day. Europe and the Middle East latitudes will see a close pairing at sunrise on Friday November 5th, while the Americas will see the pair within a five degree field later the same morning.

The Astro-word of the week is: Parallactic Inequality. This is a factor that must be accounted for in the wonderfully complex motions of our Moon. This variation is caused by the parallax of our Sun and causes the Moon to be about 2’ 7” ahead of its predicted position around 1st Quarter and lag 2’ 7” behind around Last Quarter… to give you an idea how tiny an amount that is, the apparent size of a Full Moon as seen from the Earth is about 30’. Simon Laplace once theorized that measurement of the Parallactic Inequality of the Moon’s position would serve as an effective measurement of the Sun-Earth distance, without having to chase down infrequent transits of the planet Venus from barren parts of the globe. However, one would have to know the respective masses of the Earth and the Moon to a high degree of precession for this method to be effective, a tall order in 18th century science!

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