October 20, 2017

The Moon joins a Planetary Three-Way.

Early risers this week will awaken to a fine sight; a three way dance between Mercury, Saturn, and Venus, joined by the waning crescent Moon on the 16th. Look towards the east, about a half hour before local sunrise. Mercury has just passed greatest elongation on the 6th of this month, and thus will be swiftly sinking back to the horizon morning by morning. Venus will continue to shine high in the east at dawn, and Saturn, fresh from conjunction with the Sun last month, will be the faintest and slowly slide upwards past the pair of inferior planets this week.

All three will then be passed in the opposite direction by our own Moon, making for one of the best large multi-conjunctions this year! Be sure to wake up early this AM to watch the orbital ballet unfold!

This week’s astro-term for the week is airmass. This is a term that astronomers use to describe the thickness of atmosphere that their instruments must pierce. This scattering and the atmospheric extinction results in a reduced brightness of astronomical objects and a general worsening of seeing as you near the horizon. All four objects in this weeks’ aforementioned conjunction will suffer from this effect. In fact, many observatories only observe objects when they are near the zenith or transiting to assure a minimal interference from airmass, and older style instruments such as the Rosse Leviathan were exclusively transit instruments. At Earth sea level, airmass at the zenith under clear conditions is described as 1 while airmass at the horizon is rated at 38. You can also have an airmass less than 1 at high altitudes or on mountaintops. Airmass is relative and described quantitatively as per the Beer-Lambert-Bouger Law.

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