By Calvin L. Chrisman
Jupiter, the king of the planets, named after the king of the Roman gods, will be missing from our night skies very soon. March offers the last chance to see this bright planet as it follows the Sun across the sky at dusk. As Jupiter is sinking in the western sky, Mercury, the smallest planet will be rising, but not for long. Look to the west shortly after sunset and you should see the two planets. Giant Jupiter will be the brighter of the two and will be closer to the horizon each evening as the days progress. It will eventually be lost in the Sun’s afterglow by late in the month. Tiny Mercury will continue to rise higher in the sky reaching its highest elevation on the 22nd. By month’s end, it will have faded considerably and will be difficult to spot.
Jupiter will next appear in the east, rising after midnight by August. It will be high in the south near sunrise in August. This cycle of Jupiter disappearing in the west and re-appearing in the east repeats itself approximately every thirteen months. This may seem strange since it takes Jupiter almost twelve years to complete one orbit of the Sun. This change of its position in the sky is substantially affected by the Earth’s annual orbit of the Sun which changes our perspective on this outer planet. As you look at it, it is amazing to think that, even though Jupiter appears as only a bright point of light, it is 318 times the mass of the Earth. Its distance of 5.2 times farther from the Sun than we are accounts for this perception of small size. As of this writing, we know of 62 moons orbiting Jupiter and a faint ring system that is not perceptible from Earth.
Mercury, named after the Roman messenger god, is the smallest of the planets. Its surface resembles our Moon, being covered by impact craters, lava flows and broken fault lines. It is only about six percent of the mass of the Earth and is so small that both Jupiter and Saturn have moons that are larger. While Jupiter takes almost twelve years to orbit the Sun, Mercury does so in 88 days.
The orbits of most of the planets can be correctly predicted by Newton’s laws of motion. Not so with Mercury. In the 1800’s scientists realized that something was amiss when their calculations did not match their observations. This mystery was solved in 1915 when Einstein’s General Theory of Relativity was applied to Mercury. The planet is so close to the Sun’s massive gravitational field that its orbit is affected by the ‘warp” in spacetime caused by this field. This warping of spacetime will not affect your viewing pleasure as you look at Mercury in March. Mercury will next appear in the morning sky in late December.