Is such motion even real? Should you fear Mercury retrograde? You can also check out our infographic , where we explain how retrograde motion works. Day to day and week to week, as the Earth revolves around the Sun, the planets in the sky typically move in the same direction as the Sun — from west to east. Astronomers call it direct or prograde motion.
Mars repeats its retrograde motion every 26 months, but each retrograde episode traces a slighty different path, so there is no way to predict in advance how your graph will look. It might be a loop to the left, a loop to the right, or merely a zig-zag. Here is an mp4 animation of 4 consecutive retrograde episodes of Mars.
Remember when watching that the retrograde phases occur every 26 months, so this is speeded up eight and one half million times. In this exercise you to plot and graphically observe the retrograde motion of Mars during the period from October to May , and it will allow you to see how it can be explained by both a geogentric and a heliocentric model.
Although drawing the graph is an important part of the exercise it is not the promary objective. While working on this exercise you should think about what the graph represents the actual motion of Mars through the sky during one particular retrograde episode , and also how you might explain this motion. Try your best to keep your prejudices e. You will probably find it difficult to do so! Table 1 gives the actual location in the night sky of Mars for at ten day intervals in - 71 when retrograde motion occurred.
The numbers in table 1 are the coordinates on the small section of the celestial grid in which the motion occurred. Right ascension and declination are coordinates on the celestial sphere.
Right ascension is measured in hours and minutes west of the vernal equinox, declination is measured in degrees and minutes from the celestial equator. Click for more information about graphing. Click here for the coordinates in decimal form for plotting in Excel or other graphing software. Download an Excel spreadsheet with the decimal coordinates and here for instructions on how to draw the graph using Excel also included on the Excel spreadsheet.
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Table of Contents Subscribe Digital Editons. A chronicle of the first steps on the Moon , and what it took to get there. The Magazine News Observing. Photos Videos Blogs Community Shop. Sign up! We call this retrograde motion. The retrograde motion continues for a short time and then the motion switches back to prograde. This seemingly strange behavior is easily understood within the context of a Sun-centered heliocentric solar system. The explanation for retrograde motion in a heliocentric model is that retrograde occurs roughly when a faster moving planet catches up to and passes a slower moving planet.
How the planet Mars would appear to have both prograde then retrograde then prograde motion is shown in the diagram below. Notice that it is all due to the fact that the Earth moves faster in its orbit than does Mars.
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