WHEN MERCURY'S ORBIT APPEARS "EDGE-ON" FROM EARTH

This is important for determining  Mercury's transits of the Sun.
Image Credits: NASA/JPL Solar System Simulator developed by: David Seal

EARLY MAY            FOV:  60°       Magnification:  0.5x (to show entire orbit)       Inner Planet Orbits

L O A D I N G 

Unlike the Superior Planets, the Inferior Planets—Mercury and Venus—have orbits smaller than Earth's.  This makes it possible to depict, in a single view, how each one's full orbit would appear from Earth at any instant.  In addition, as Mercury's and Venus's orbits are tilted relative to Earth's orbit, both change in "apparent" shape day after day, as Earth circles the Sun.  Most often, if you plotted the shapes of Mercury's and Venus's orbits, they would appear to us to be a very narrow ovals that "wobble" around the Sun as Earth progresses in its orbit.  But twice each year both Inferior Planets' orbits shrink in thickness to become lines, as we see them "edge on" from Earth.

Though these dates change very slowly over the span of many human lifetimes, viewed from Earth in the current era Mercury's orbit appears "edge-on" to us twice each year: in early May and early November.  The views directly above and below show this.  Mercury's orbit is the white-ish line segment slightly narrower than each view's overall width.

EARLY NOVEMBER            FOV:  60°       Magnification:  0.5x (to show entire orbit)

The next view—looking down on the inner Solar System—helps us understand why Mercury's orbit "flattens into a line" twice a year from our perspective here on Earth.  In early May and early November each year, Earth's motion around the Sun carries it through the plane of Mercury's orbit, which is tilted 7.00° to the Ecliptic.  These same cir- cumstances may be expressed in several ways.  For example, it can be said that early May and early November are the times when Earth crosses Mercury's line of nodes, the line of intersection of the planes of Mercury's and Earth's orbits.  It can also be said that these are the times when Mercury's line of nodes "points directly at" Earth!  These three descriptions are essentially equivalent, and they are important because Mercury's orbit is smaller than Earth's.

If Mercury is close enough to a node at the proper time in early May or early November, such that it passes directly between Earth and the Sun, then a  transit  will occur, i.e. the planet will be seen to move across the Sun's disk!  At all other times, Mercury appears too far from the Sun for a transit to occur.  Because both Earth and Mercury must be in specific areas of their own orbits at the same time to permit these events,  Mercury's transits of the Sun occur rarely, only about 13 or 14 times each century!  List  These tend to happen in pairs, the constituents of a pair sepa- rated by a few to several years.  Succeeding pairs alternate in a varying cycle of intervals.  Mercury's last transit of the Sun occurred on  2019 November 11  , while  its next one will occur on  2032 November 13.  Images opened by the following two links reveal the different geometries of Mercury's  May Transits  versus its  November Transits. 

With the selectable bonus views directly below you can observe how  the orientation of Mercury's orbit appears to change as seen from Earth throughout the year.  Just click through the months or on the month you want to view.  It is easy to see that  Mercury's distance from the Sun can vary greatly in its significantly elliptical orbit.  This is why Mercury's November transits occure more frequently than it May transits.

 

BONUS: ORIENTATION OF MERCURY'S ORBIT BY MONTH      FOV:  60°    Magnification:  0.50x (to show entire orbit)

The  Mercury & Venus 2013-2014 video  confirms the changing orientation of Mercury's orbit throughout the year.  And don't forget to also check out  When Venus's Orbit Appears "Edge-On" from Earth.

Simulator Image Notes:  1) Fields of View are measured horizontally.  2) Orbital views' orientation: Sun's local north "upward", i.e. in the
12:00 midnight direction.  3) Light-Travel Time is not included in calculated times.

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