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I have been messing around with the elongation for Mercury, and I have been getting 27.8 degrees at maximum in https://www.heavens-above.com/PlanetSummary.aspx. Is there any history record that Mercury could go higher than 27.8 degrees or is 27.8 degrees the absolute maximum elongation that could be ever achieved?

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    $\begingroup$ The title asks about altitude, the text asks about elongation. You should clarify which you're interested in. $\endgroup$ Commented Sep 27, 2022 at 21:23

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Yes, 27.8 is the maximum that can be achieved. jgiesen.de calculates this from the eccentricity of the orbit of Mercury:

$$\sin β = a(1+e)/(1\, \text{AU})= 0.467,\ β = 27.8°$$

For Mercury a=0.387 and e=0.206.

You could make a small correction for the Earth's eccentricity, but this is very much less than the eccentricity of Mercury, so has less effect

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