Spot Mercury at Dawn

Most skygazers have never seen the planet Mercury because it never strays very far from the sun. This week is one of the rare opportunities to catch Mercury just before sunrise.

On the morning of Tuesday July 30, Mercury will be at its furthest from the sun, just under Jupiter and Mars. Credit: Starry Night software

Two things about Mercury make it a tough target. First and foremost, it never strays far from the sun because of its location in the inner solar system. It is always observed against a bright twilight sky, either half an hour after sunset or half an hour before sunrise, and always low in the sky.

Secondly, it is a tiny object. Now that Pluto has been demoted to "dwarf planet" status, Mercury, at 1550 miles (2440 km.) in diameter, is the smallest of the eight planets, 38 percent of Earth's diameter.

The challenge in observing Mercury is to find a tiny speck of light, low in the sky, against bright wilight. This week offers one of the rare opportunities when conditions are at their best.

Mercury will be farthest from the sun in our sky on July 30. The weeks just before and after this date are equally favourable.

The best time will be about 40 minutes before sunrise, a balance between Mercury's altitude above the horizon and the brightness of twilight. Scanning the sky with binoculars will help to spot the tiny speck of light. Once spotted in binoculars, you should just be able to see Mercury with the unaided eye.

Because of Mercury's low altitude, a low cloudless eastern horizon is necessary. The task is made easier by the presence of two much brighter planets in the dawn sky, Jupiter and Mars. Mercury will be below and to the left of these two objects.

Once you spot Mercury, locate it relative to landmarks on your horizon, and see how high you can follow it as it rises and the sky becomes brighter. If you have a telescope, this task becomes easier.

Seen in a telescope, Mercury will look like a tiny gibbous moon. As it rises higher in the sky, it will clear the turbulence, and the view will improve. have succeeded in observing one of the most elusive objects in the sky. It is said that the great 16th century astronomer Johannes Kepler never managed to see Mercury.

Mercury and the Solstice

The last few weeks have provided an unusually fine opportunity for skygazers to spot the elusive planet Mercury because it has been in close proximity to brilliant Venus, and, earlier, Jupiter as well.

On the evening of Thursday June 20, Mercury reaches its maximum elongation east of the sun, placing it directly below brilliant Venus in the evening twilight sky.  Credit: Starry Night software

This opportunity is now coming to a close as Mercury passes its maximum elongation from the sun on Thursday June 20 and begins its rapid drop towards the horizon, passing between us and the sun on July 9. For the next few nights, Mercury will be a tiny speck just below Venus. It is closest on June 20, slightly less than two degrees away, but will also be very close one night earlier or later.

The best time to see Mercury is about half an hour after local sunset. Any earlier, and it will be lost in the sky’s glare; much later and it will be too low to make out. It is most easily spotted with binoculars, but once you’ve located it, it should be relatively easy to see naked eye.

This week also marks the solstice, on Friday June 21 at 1:04 a.m. EDT. The sun reaches its most northern declination, marking the middle of summer in the northern hemisphere and the middle of winter in the southern hemisphere.

Because the sun is as far north as it can get, it is above the horizon in the northern hemisphere as long as is possible. Also, at local noon, it is as high in the sky as it can get. These two factors bring the maximum solar heating possible.

In the southern hemisphere, the opposite is true. The day is as short as it can get, and the sun is low in the northern sky, giving little warmth.

If June 21 is the “midsummer” or “midwinter” day, why is it that we always think of the seasons as beginning on this day? It’s because it takes time for the sun to have its effect, causing the seasons to lag behind the sun, so that the hottest days of summer (or the coldest days of winter) are usually a month or two after the solstice.

The solstices have always been important dates for humans. Most calendars mark the beginning of the year close to the winter solstice. Determining the exact date of the solstice was important to fix the calendar, and structures like Stonehenge in England were built to make accurate measurements of the sun’s rising and setting points.