How is it possible that Mercury can be seem from Earth if it is such a small planet, it has the lowest albedo of any planet, and it is so close to the Sun?


1 Answer 1


With an albedo of 0.09 (Mallama 2017), it's true that Mercury has the lowest albedo of all planets. But reflecting 9% of something that is really, really, really bright, is still pretty bright.

Comment from Darth Pseudonym: For comparison, the Moon's albedo is 0.12 on average, so Mercury reflects almost as much as the Moon.

Being close to the Sun is a problem. Mercury is brightest when almost behind the Sun so that it is "full" and we see the full reflecting face of the planet. Moreover, the albedo is actually larger in this configuration, because planets tend to reflect light preferentially back in the direction of the illuminating source. However, in this case it would be daytime, as seen from Earth, so that the Sun outshines Mercury (at least to the naked eye; you can in fact see it in a telescope, but be careful not to catch the Sun).

Therefore Mercury is best observed roughly at "half-Mercury", at which point according to Wikipedia its separation from the Sun is roughly 18° to 28°. This puts Mercury well into the twilight and even night, although you do need a rather unobstructed view toward the horizon, as its altitude will be correspondingly low.

Based on the phase, distance, and albedo you can then calculate the apparent brightness (e.g. by following the steps in this answer), and you'll find that Mercury's magnitude alternates between roughly 6 (barely visible) and –2 (brighter than Sirius; the brightest star on the sky). Even in twilight, seeing the brightest stars is no problem, unless you're in the middle of a large city.

Here's a diagram (generated from this site) showing how the apparent magnitude changes throughout the coming year:


Comment from PM2Ring: You can compare this to Mercury's separation from the Sun to see when it is best observed. Here's a plot of their ecliptic longitude and latitude for the next three months, where numbers are given for every 7 days:

enter image description here Credit: User @PM2Ring.

  • $\begingroup$ It's worth noting that the moon's albedo is 0.12 on average (it varies wildly because moondust has a retroreflective property) so Mercury isn't THAT much darker than a half-moon... $\endgroup$ Commented Jan 2 at 16:18
  • $\begingroup$ Here's a plot of the ecliptic longitude & latitude of Mercury & the Sun, for 2024-Jan-1 to 2024-Apr-1, with numeric labels every 7 days. i.sstatic.net/6JyJ7.png $\endgroup$
    – PM 2Ring
    Commented Jan 2 at 17:53
  • $\begingroup$ @DarthPseudonym Yes, good point! $\endgroup$
    – pela
    Commented Jan 2 at 20:44
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    $\begingroup$ @PM2Ring Thanks, that's also very informative! $\endgroup$
    – pela
    Commented Jan 2 at 20:49
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    $\begingroup$ @pela You're more than welcome to use my plot! $\endgroup$
    – PM 2Ring
    Commented Jan 5 at 9:28

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