When is Mercury visible?

Observe Mercury

Finding Mercury in the sky is not easy. It is the planet that orbits the sun most closely and never strays far from it. That is why it can only be seen in their vicinity, i.e. at dawn or dusk. He can never be seen at night. It doesn't set long after the sun sets (or rises some time before it).

Mercury's orbit runs within the earth's orbit. That is why Mercury overtakes us every now and then. It moves fairly quickly and during the visibility phase it stands a little bit shifted compared to the previous day in the starry sky every evening (or morning).

Mercury on its orbit

If Mercury is to the left or right of the sun (as seen from Earth), we can see it at dusk.

However, it must be far enough away from the sun so that it is not outshone. It only becomes visible when the sun is below the horizon (i.e. has already set or has not yet risen).

These phases of visibility often only last a few days to two weeks, then it disappears again in the light of the sun.

If Mercury is on its orbit (as seen from us) in front of or behind the sun, it is invisible to us.

Even if it is still too close to the sun, we cannot see it. It just disappears in the glaring bright sunlight.

Whether Mercury can be observed at the moment can be found on the observation page for the current month. Otherwise it is best to ask an observatory when Mercury can be seen. Or get yourself a guide to the sky, we recommend the yearbooks 'Kosmos Himmelsjahr' or 'Ahnert's astronomical yearbook'. It is not that easy to find Mercury in the sky. In some months it remains completely unobservable.

Here is another observation tip for professionals: Mercury is easier to observe in the evening sky in spring, but in the morning sky in autumn. Of course, this only works if Mercury is currently on the 'right' side of the sun, i.e. in spring it only sets after the sun and in autumn rises above the horizon before the sun.

On the other hand, it is difficult or impossible to find it in the morning sky in spring, and it is hardly to be seen in the evening sky in autumn. This is due to the position of the ecliptic in these seasons.

Mercury in the sky

It would look something like this in the evening twilight sky - the sun is already below the horizon. In the increasing darkness, Mercury becomes visible. But soon it will set too, because it follows the sun.

Mercury is never far from the horizon. In the photo we see Mercury (right) and Jupiter. Jupiter appears a little brighter than Mercury for three reasons: it is further away from the horizon, it is larger, and it has a higher albedo.

Mercury's light phases

Mercury also shows the change of light phases as we know it from the moon. To see this, you need a telescope.

Depending on its current position and the position of the earth (our observation point), Mercury appears to be illuminated differently. If the sunlight comes from the side, Mercury appears like a crescent moon. As with the moon, there are the phases full mercur, half mercur, and all the gradations in between.

The size of the Mercury disc is not always the same either. If it is close to the earth, it is larger. In a position at the rear of its orbit, it appears very small to us.

Illuminated area and current size together result in the luminosity of Mercury. Sometimes it shines quite brightly, sometimes it can hardly be seen. The brightness fluctuates between -1.9 mag and 2 mag.

If it moves towards the earth on its orbit, its disc becomes larger, but less illuminated, as we are increasingly seeing it 'from behind'. If it has overtaken the earth inside and moves away, it becomes smaller, but the degree of illumination increases.

Since Mercury is an inner planet, from time to time it overtakes the earth, moving between the sun and the earth in the process. One might think that Mercury would then move across the solar disk (as seen from Earth) every time, but that is not the case. Earth's orbit and Mercury's orbit are not exactly in one plane, but are slightly tilted against each other.

They only cross at two points, the 'knots'. And only when Mercury is in the immediate vicinity of a node and it overtakes the earth does the so-called Mercury transit occur, the crossing of the sun through the nimble planet. There is more about this on the 'Mercury Transit' page.

Tags: Mercury, observation

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