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• End of nautical twilight and Mercury's setting time in your sky
• We should mention that the Northern Hemisphere enjoys the much better view of this particular evening apparition of Mercury. That's because the ecliptic - the pathway of the planets - hits the horizon at a steep angle as the sun sets in the Northern Hemisphere but at a shallow angle in the Southern Hemisphere.
• What is the ecliptic?
• Mercury stands higher over the horizon at sunset in Northern Hemisphere than at comparable latitudes in the Southern Hemisphere. For instance, at 40o north latitude - the latitude of Philadelphia, Pennsylvania - Mercury's altitude at sunset is about 16.5o. In stark contrast, at 40o south latitude - the latitude of Wanganui, New Zealand - Mercury is less than half as high at sunset, at close to 7o altitude.
• No wonder Mercury sets more than 90 minutes after sunset at mid-northern latitudes but less than 40 minutes after sunset at mid-southern latitudes. The farther north you live, the later that Mercury sets after sunset; and the farther south you live, the sooner.
• If you live in the Northern Hemisphere or the southern tropics, you have a reasonably good chance of catching Mercury. Look for Mercury above the sunset point on the horizon some 45 to 60 minutes after sunset.
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• At northerly latitudes, Mercury should be visible in a clear sky for another week or so.