There are several factors to consider in determining which planets can never be seen at opposition. In this article we will talk about their geocentric longitudes appearance cycle and apparent stationary point. We will also look at their orbital declination. This information will help you determine the exact position of the planets during the opposition and conjunction phase of the Moon. Once you know this information you can make your own observations.
Inferior planets
It is impossible to see an inferior planet at opposition because they never reach their maximum elongation. Their maximum elongation is determined by their latitude and season as well as the angle of the ecliptic to the observer’s horizon. In addition inferior planets are rarely seen during the night. But they can be seen during twilight.
Their orbital declination
Planets at opposition are at their closest point to Earth’s position in their orbit. There is a slight difference in the closest point due to their non-circular orbital shapes. Jupiter’s opposition was July 14 2020 and the closest point was July 15 2020. Planets at opposition are the most visible and will remain bright during several weeks around this time. A telescope can help you see them at opposition.
Their appearance cycle
Mars and Earth are in regular orbits around the Sun. They pass over each other 7 times over a 15.8-year cycle. During the time between their appearance cycles they have similar positions on the ecliptic. While Earth travels over Mars for approximately 2 years Mars travels 50 days. During the time between their opposition positions the two planets appear to be in a parallel orbit. In the shortest cycle Mars and Earth will appear to pass over each other once every two years.
Their apparent stationary point
Mars and Jupiter appear to travel in a circular orbit. When the planets pass the same stationary point their motion slows down and speeds up. The observed motions of Mars and Jupiter are shown in figure VIII.9 and the motions of asteroids are also shown. The motion of Mars and Jupiter varies in speed and the elongation of their apparent stationary point is smaller than the greatest elongation.
Their length of apparition
Planets make a brief apparition at their closest point to Earth when they are in opposition. The planets’ apparitions vary depending on the type of planetary system. In general inferior planets orbit closer to the Sun than the Earth so their apparitions last for a shorter period of time. The occurrence of the planets at opposition is less dramatic than the apparitions of larger brighter planets.
