Everyone asks “when is Mercury retrograde?” but no one ever asks “how is Mercury retrograde?”
All objects in the sky move East to West during a night due to the West to East rotation of the Earth. Retrograde motion is when a planet moves East to West relative to the stars.
Looking down on the solar system from the North, prograde motion occurs when the line from the Earth to the object rotates counterclockwise (in the same way the planets actually move). Retrograde motion occurs when the line rotates clockwise because the Earth passes the planet. Hence, retrograde motion only occurs if the Earth is moving faster than Mercury. Mercury will appear to move backwards when we pass it. This is possible because of Kepler’s second law of panetary motion, which states that planets can move at different speeds during their orbit depending on their proximity to the Sun.
Retrograde motion is illustrated nicely in the video at the top of this article.
There is famously no closed-form solution for calculating the trajectories of orbit in a complex systems. We use an ephemeris which defines precomputed metrics of interest.
Ephemeris (plural: ephemerides): A table or data file giving the calculated positions of a celestial object at regular intervals throughout a period.
The primary metric from the ephemeris we use with regard to Mercury is its Right Ascension (RA). According the the Americal Institute of Physics, RA is the astronomical equivalent of longitude. More specifically, it is the angular distance of an object measured Eastward from the Vernal Equinox.
Most importantly: when Mercury’s RA is decreasing over time, it is retrograde. This means that it is moving backwards across the sky. The server uses skyfield to calculate this, by computing Mercury’ RA in comparison to Earth’s.
For a more detailed description of this concept, see Montebruck & Pfleger’s book from 2000, Astronomy on the Personal Computer (Fourth, Completely Revised Edition). For more basic information on how ephemerides are calculated, go to the NASA Expolanet Archive.
No.
Well, sure. Language need only be understood, and most people would understand that phrase. However, strictly, this is a colloquialism and grammatically incorrect in formal contexts. This is because “retrograde” is treated as an adjective rather than a noun.
This would be a nice feature to add, however, I have not yet had the time to figure out how this would be calculated (sure, brute force would work, but there must be a better way). For a more popular alternative with this feature, see Kate Trgovac’s “Is Mercury in Retrograde?”.
See also: Is Mercury retrograde?