Relativity Lite

Gravity Lite | 49 One other significant understanding comes from figure 3. The warping of spacetime around the Sun is the cause of the Earth moving around the Sun rather than flying off in the straight line its momentum would otherwise produce. This is much like the orbits a coin makes in the March of Dimes funnels one finds in public spaces. In this fashion, Einstein has replaced the idea of Newton’s gravitational force with a more fundamental explanation of events that predicts the motion of light as well as massive bodies. One may find the limit of Einstein’s equations in the case of a small warping of spacetime, such as that caused by the Sun in the vicinity of the Earth, and the results agree perfectly with Newton’s “force.” But when gravitational effects become stronger, Newton’s descrip- tion becomes more and more inaccurate, whereas Einstein’s accurately predicts the state of affairs. Another key finding of Einstein’s 1915 paper was an explanation of the shift in the point of closest approach of the planet Mercury to the Sun, its perihelion . Since the 1850s astronomers had been trying to explain an anomalous extra 43 seconds of arc per century by which Mercury’s perihelion advances. In figure 5, the blue rosette shows this advance of the perihelion, much exaggerated. Figure 5. NASA and MIT scientists analyzed subtle changes in Mercury’s motion to learn about the Sun and how its dynamics influence the planet’s orbit. The position of Mercury over time was determined from radio-tracking data obtained while NASA’s MESSENGER mission was active. Credit: NASA’s Goddard Space Flight Center. * One may understand why Mercury’s perihelion advances by looking at a diagram of what is called the potential energy (PE) of its orbit. Potential energy is best understood by * Elizabeth Zubritsky, “NASA Team Studies Middle-Aged Sun by Tracking Motion of Mercury,” National Aeronautics and Space Administration, last modified January 18, 2018, https://​www​.nasa​.gov/​feature/​goddard/​2018/​nasa​-team​-studies ​ -middle​-aged​-sun​-by​-tracking​-motion​-of​-mercury.