My Dark Sky

Of the five planets known since ancient times, Mercury is the most difficult to see. Because it is the closest planet to the Sun, it always appears in the direction of the Sun in our sky. It can never appear more than 28 degrees away.

Since Mercury is always near the Sun, it can either be seen in the west just after sunset or in the east just before the Sun rises. Not knowing that these two are actually the same objects, the ancient Greek called it Hermes (or Mercury by the Romans) when it appears in the evening and Apollo when it appears in the morning sky. Mercury moves between the morning and evening skies about half a dozen times each year.

The inferior planets (Mercury and Venus) go through phases just like our Moon because their orbits are between the Earth and the Sun. When these planets, the Sun and the Earth are in a straight line with the planets on the other side of the Sun, we have a superior conjunction. This is the time when we have “full” Mercury or Venus. However, the planets are in the glare of the Sun and thus are not visible.

As they move out behind the Sun, most of their illuminated surface can be seen from Earth, hence they are gibbous phase. As they reach greatest eastern elongation, half of their surface can be seen, hence quarter phase. After that, they will be moving toward the point between the Sun and the Earth – toward the inferior conjunction. Their phase will become crescent and eventually when they reach inferior conjunction, they again lost in the glare of the Sun.

From superior conjunction to inferior conjunction, the angular size of the planets is increasing because they are moving toward us, but their magnitudes are decreasing because lesser and lesser surface are illuminated from Earth point of view.

After inferior conjunction, as the planets move toward greatest western elongation and toward superior conjunction, the opposite applies. The inferior planets are now visible in the morning sky, before the Sun rises.

This month, Mercury will be an evening “star”. It had just coming out behind the Sun and heading towards its greatest eastern elongation. When inferior planets reach their greatest elongation – eastern if visible after sunset and western if visible before sunrise – they are at their maximum angular distance from the Sun in the sky and this is the best time to try to spot them, especially true for Mercury. Greatest elongation for Mercury varies between 18 degrees to 28 degrees due to its elliptical orbit and orbital inclination.

As Mercury emerges behind the Sun, it is gibbous in phase, shining at magnitude -0.9. When it reaches its greatest eastern elongation on May 13, lying 22 degrees from the Sun, its phase changed to quarter, and its magnitude drop to 0.4.

After that, Mercury will “make a U-turn” and heading back toward the Sun again. Its illuminated surface will continue to decrease – getting more and more crescent and fainter – as it moves between us and the Sun. Eventually, it will disappear from our view again below the western horizon by end of the month.

Bonus on May 7: a very thin crescent Moon is about 7 degrees to the northwest of Mercury, barely fit into the same field of view of a binoculars.  Click image to enlarge.

Mercury is in Taurus now, not far away from Pleiades or “Seven Sister” but is pulling itself away from the cluster. Mercury is easily visible to the naked eye, provided you have a clear and unobstructed west-northwestern horizon. Binoculars and telescopic view can also be very rewarding. You can follow the phases of Mercury as it thinning to a crescent and growing bigger at the same time.

Let’s go and catch this elusive planet!

This image was taken in Kuala Lumpur in year 2005 during the conjunction between Mercury, Venus and Saturn in the evening sky.  Credit: Tang KH (APGM) 

~ by thChieh on May 6, 2008.

Posted in Observations & Tips