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Why did Mercury not appear to transit through the middle of the sun (the equator)? I initially thought that this was because of the ecliptic, but, then I thought that actually we'd see it above the solar equator, wouldn't we? So I'm confused.

This video of the transit from NASA shows exactly what I'm talking about. Mercury is shown relatively low on the solar disk.

I'm looking specifically for why it was visible in the bottom half of the sun, not the top half?

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    $\begingroup$ Because, Earth, Mercury and the sun were not all in a perfect line. (If Earth and Mercury's orbits were not inclined - i.e. were in a perfect flat plane - we would see several transits per year in fact.) $\endgroup$ – Andy May 11 '16 at 7:24
  • $\begingroup$ Why did you think "that actually we'd see it above the solar equator, wouldn't we" ? Did you not wonder why this is not seen every Mercury year? Most Mercury years, it does not pass in front of the Sun at all (for some good reason explained in the answers), so when it does, why should it go through the equator? $\endgroup$ – Walter May 13 '16 at 21:53
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Mercury's orbit is inclined 7 degrees relative to Earth's orbit.

This drawing is exaggerated. When Mercury is "between" us and the Sun it might be below, above, or passing through the Sun's disc from our point of view.

enter image description here

Another post from Universe Today explains this also.

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    $\begingroup$ I don't think this explains the observation. Though the fact appears right, to be seen Mercury had to be crossing the plane of the ecliptic so its orbital inclination seems unimportant. $\endgroup$ – Software Engineer May 10 '16 at 23:54
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    $\begingroup$ The set of positions that would make Mercury appear in front of some portion of the Sun is not a perfect plane. It is a swath of some width. The fact that Mercury's orbit is inclined 7 degrees explains how it could deviate from being perfectly aligned with the Equator of the Sun during its transit. Sometimes it might be perfectly lined up, sometimes it will be too far from the ecliptic so that it won't transit the disc of the sun at all, sometimes it will transit the Sun off-centre. $\endgroup$ – user10250 May 11 '16 at 0:08
  • $\begingroup$ Yes, but why, specifically, was it seen in the lower half of the sun this time? With an inclination of 7 degrees relative to the earth I would have expected to see it in the top half. I'm looking for an explanation of why I didn't see that. $\endgroup$ – Software Engineer May 11 '16 at 12:19
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    $\begingroup$ Your expectations are false, an inclination of 7 degrees means Mercury spends half its time above and half its time below the celestial equator, there is no preferred "half" of the sun it should cross, it was just by chance that it crossed where it did this time. $\endgroup$ – Dean May 11 '16 at 14:51
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As Mercury is inclined by 7 degrees relative to the orbit of the Earth (the ecliptic) at any given time it may be above the ecliptic or below. However there are two times during its orbit when it crosses the ecliptic. These are known as "nodes".

A conjunction occurs when the planet is vertically above or below the sun. A superior conjunction happens when the planet is on the far side of the sun, an inferior conjunction happens when the planet is closer to the Earth than the sun.

When a node occurs at about the same time as an inferior conjunction, then the planet will appear to travel in front of the sun. Whether it travels through the middle of the solar disk, or through the upper or lower part, depends on whether the exact time of the node occurs just before the conjunction or just after.

In May 2016, Mercury crossed the ecliptic at about 08:00 (utc), and had it inferior conjuction at about 15:00. It was a little below the ecliptic by the time of conjunction, but still close enough to cross the solar disk.

The exact position of Mercury also depends on the location you are viewing from on Earth, a fact that Edmond Halley noticed could be exploited to calculate the distance to the planet, and by extension, the distance to the sun.

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