High Mercury

.....I'm not referring to the temperatures getting hotter as we approach the Summer Solstice, but the planet Mercury, which is becoming visible in the west after sunset, although I might be overstating that just a bit. Of the visible planets, Jupiter and Saturn are easy to find. Saturn is still bright in the southern sky as it gets dark, and Jupiter is bright in the sky before dawn (slowly moving into the evening sky; by September, Jupiter will be replacing Saturn in the evening). Mars is basically visible every other year, since it is only a bit farther away from the Sun as the Earth, so when the Earth passes Mars, it takes a long time to "lap" it again. Venus, like Mercury, is either a morning star or an evening star, but it is much easier to see (explanation to follow).

.....Mercury (and Venus) orbits the Sun on a smaller orbit than the Earth has. This means that Mercury (for example) can never be high in the sky at midnight because it would then have to be exactly opposite the Sun in the sky. This is clearly impossible. Taking this to a less extreme case, we can see that Mercury will always need to be close to the Sun in the sky. (Mercury also has a very elliptical orbit as well - the most elliptical orbit by far of the official planets.)

.....As you can see from the above view, Mercury is never more than 28° from the Sun in the sky at the best of times, which makes viewing Mercury very difficult. (for the nest few weeks, we are at about the middle between these two extremes.) The Sun is so bright that nothing but the Moon can be seen in the sky with the Sun, so to see a planet, we need the Sun to be below the horizon. That means that Mercury can never¹ be seen higher than 28° above the horizon, but it gets worse ...

.....The sky does not suddenly go dark when the Sun goes below the horizon because the light from the Sun is scattered in the atmosphere, so we have to wait longer, for the sky to darken, in order for Mercury to be visible. Astronomical twilight lasts until the Sun is 18° below the horizon, so in the best of times Mercury is only 10° above the horizon, while if Mercury is observed at its closest to the Sun, then Mercury will be setting at dark. Even in this best case, this means that we will be looking at Mercury through a heck of a lot more air. (We don't have to wait for the sky to be completely dark; Mercury is bright enough to show up while there still is some light in the sky, but it can be challenging to see.

.....When looking at an object 10° above the horizon, we are looking through 5.6 times as much air as when something is directly overhead. This would be like setting up your telescope at the bottom of a swimming pool - and it keeps getting worse.

.....The ecliptic is the path of the Sun across the sky. The planets have to stay near (though not on) this line, so at times when this line makes the greatest angle with the horizon (for the Northern hemisphere, this would be March), we have a better chance to see the planet. In June and early July, we are closer to the condition on the right above, than the left.


.....Until the first spacecraft went to Mercury (Mariner 10, in 1972-73), or the second (MESSENGER, in orbit as of this last March), views of Mercury were difficult to get. We had these really bad views, or wait for a solar eclipse (you guys check the footnotes, right?), so the best diagrams looked like those below, a sketch made by the astronomer Giovanni Schiaperelli. This does not look like satellite photos of Mercury (unavailable until 1973, and shown at the right). The sketch below looks more like a captcha than a planet!

Jello?

.....To see Mercury in the next few weeks look close to the horizon just after sunset. For example, tomorrow night, Mercury should appear at about 8:55 when it is about 7° above the horizon. This is less than the angle between your fingers when making the Arachnid Super Hero gesture / secret devil sign (depending on your taste in seventies metal) at arms length. You can see that if you have a wooded or hilly horizon, you are kind of hosed. The best case scenario will be on July second starting at about 9:05 when Mercury will be 10° above the horizon. Woo hoo!

1: Or during a solar eclipse, but that seems like an uncommon an extreme caveat to toss in.