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Who will see Mercury before sunrise?

Mercury – the innermost planet – swings to its greatest western elongation from the sun (27o) in late April 2018. Because Mercury is farthest west of the sun right now, that places Mercury in the eastern sky before sunrise. For the Southern Hemisphere, this is Mercury’s best apparition in the morning sky for all of 2018; yet at northerly latitudes, this is Mercury’s poorest showing in the morning sky for the year.

By the way, our friends in the Southern Hemisphere will be thrilled to know that the feature sky chart above is especially for their part of the world. It’s specifically for Cape Town, South Africa, which is roughly the same latitude as Sydney, Australia and Auckland, New Zealand and Santiago, Chile. Given an unobstructed eastern horizon, all these places should be able to view Mercury with the eye alone in a predawn sky! However, binoculars always come in handy for any Mercury search.

At temperate latitudes in the Southern Hemisphere (South Africa, southern Australia, New Zealand), Mercury rises better than 2 hours before the sun. At mid-northern latitudes (United States and Europe), on the other hand, Mercury comes up about one hour before sunrise. Whereas Mercury will climb over the eastern horizon before dawn’s first light for two to three more weeks at southerly latitudes, mid-northern latitudes will find Mercury deeply buried in the glow of twilight. That’s in spite of the fact that this present morning apparition will last for yet another month.

Click here for a recommended almanac giving you Mercury’s rising time in your sky.

At northerly latitudes, the shallow angle of the ecliptic – the pathway of the sun, moon and planets – on springtime mornings keeps Mercury buried in the glare of sunrise.

Some of you might be wondering why this morning showing of Mercury is so outstanding in the Southern Hemisphere but so poor in the Northern Hemisphere. After all, Mercury’s elongation of 27o west of the sun is the same for both the Northern and Southern Hemispheres. The disparity is due to the angle of ecliptic – the roadway of the planets – across the sky. The ecliptic crosses the predawn/dawn sky at an exceptionally steep angle (nearly perpendicular) in early autumn yet a particularly shallow angle (almost horizontally) in early spring.

Since it’s now autumn in the Southern Hemisphere, the ecliptic crosses the predawn/dawn sky at a steep angle.

Since it’s now spring in the Northern Hemisphere, the ecliptic crosses the predawn/dawn sky at a shallow angle

The ecliptic is actually the Earth’s orbital plane projected onto the constellations of the zodiac. However, since the planets of the solar system orbit the sun on nearly the same plane that Earth orbits the sun, the planets always reside on or near the ecliptic.

If you live in the Southern Hemisphere, these next few weeks present your best opportunity of the year to spot Mercury, the innermost planet, in the morning sky.

Bruce McClure

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