Mercury reaches its greatest elongation east of the sun this week on Friday, September 4.
On Friday, September 4, Mercury will be at its greatest elongation east of the Sun. Credit: Starry Night software.
Here we see it as we might from space, say on the International Space Station or the Hubble Space Telescope. The green line marks the ecliptic, the path the sun appears to follow over the year. Most of the planets also appear to revolve in this same plane.
The red line is Mercury’s orbit, which you can see is tilted quite a bit compared to the ecliptic. Of all the planets, Mercury’s 7 degree tilt is the most extreme. The small orange dot marks Mercury’s position on Friday, as far east of the sun as it can go.
How will Mercury look to us here on the surface of the Earth? It very much depends on where you are located.
As seen from Europe and North America, the ecliptic makes a shallow angle with the horizon, so Mercury is not well placed. Credit: Starry Night software.
This view of Mercury is how it will appear to me from my location close to Toronto, Canada at sunset. Everyone at a similar latitude across southern Canada, the northern United States, and most of Europe and Asia will see something very similar.
Because of the Earth’s current position in its orbit around the sun, the ecliptic makes a very shallow angle with the western horizon as seen from the northern hemisphere. So even though Mercury is as far west of the sun as it can get, at this time of year it ends up very close to the horizon around sunset, the best time to look for it. To make matters worse, because of its orbit’s tilt, Mercury is quite far south of the ecliptic at this time. As a result Mercury is barely 7 degrees above the horizon at sunset.
The situation in the southern hemisphere, here seen from southern Australia, is very different. The ecliptic makes a very steep angle with the horizon, and Mercury is south of the ecliptic, so Mercury is much higher above the horizon at sunset, 26 degrees in fact.
As seen from Australia and South Africa, the ecliptic makes a steep angle with the horizon, so Mercury is very well placed. Credit: Starry Night software.
As a result, spotting Mercury from the northern hemisphere will be a major challenge this week, but the lucky people in the southern hemisphere will have a fine view.
This situation reverses in the spring, when northerners get a fine view of Mercury at dusk, and southerners are out of luck. It also reverses when Mercury is at elongation on the western side of the sun. That is why every year we publish a table showing which elongations of Mercury will be favorable or unfavorable, depending on which hemisphere you view it from.
All in all, even though it is very bright, Mercury is probably the most challenging planet to view. You have to be in the right place at the right time.
Mercury viewed through a telescope is a disappointment. The most you will see is a tiny disk, which goes through phases similar to Venus and the moon. But it is a great satisfaction to most stargazers to say that they have actually seen it at all.