The angular diameter of Jupiter at opposition varies little from one opposition to the next. By contrast, the angular diameter of Mars at opposition is quite variable. Explain why there is a difference between these two planets.

When observing planets within our solar system, one of the key concepts is the 'opposition of planets'. This astronomical term refers to the event when a planet is positioned directly opposite the Sun as seen from Earth. During opposition, the planet is typically closest to Earth, making it appear brighter and more prominent in the night sky.

For students studying the night sky, understanding opposition is crucial because it represents the best time to observe planets such as Mars or Jupiter. In opposition, not only are these planets more visible, but their angular diameter, which is the apparent size of a celestial body when viewed from Earth, is also at its maximum. As a result, features of the planets such as surface markings and atmospheric phenomena can be more easily studied through telescopes.

In simple terms, opposition is much like standing directly between two friends in a straight line, and turning around to face one of them – that friend is said to be in 'opposition' to you. It's during this alignment that planets can be observed in greater detail, a valuable opportunity for astronomers and enthusiasts alike.

Observing celestial bodies, such as planets, involves more than just looking up at the stars; it's a complex process influenced by a variety of factors. One of these factors is angular diameter. The angular diameter is essentially how large a planet appears from our point of view on Earth. This visual size is measured in degrees, arcminutes, or arcseconds, with full circles having 360 degrees, each degree having 60 arcminutes, and each arcminute containing 60 arcseconds.

For example, the Moon's angular diameter is about half a degree wide on average. When students become familiar with measuring angles in the sky, they can grasp the scale and distances involved in space, providing context to their observations. Factors like a planet's size, its distance from Earth, and its position in its orbit can all affect its observed angular diameter. Recognizing these elements enhances one's understanding of celestial mechanics and aids in the prediction of optimal observation times.

Additionally, for students or anyone curious about watching the skies, knowing when a planet goes into opposition can help schedule a night of viewing for the best possible experience. This knowledge, paired with an awareness of the weather and local light pollution levels, can significantly improve the quality and enjoyment of the celestial observation.

Orbital eccentricity is a measure of how much an orbit deviates from being circular. In the solar system, no planet's orbit is a perfect circle; they are all somewhat elliptical. Orbital eccentricity can have significant effects on a planet's weather, temperature, and visibility from Earth.

Imagine the orbits as stretched-out circles or 'ovals'. If a planet’s orbit is highly eccentric, there will be a large difference between its closest point to the Sun, known as perihelion, and its farthest point, known as aphelion. This concept is linked to the variable angular diameter of planets like Mars when viewed from Earth at opposition.

Mars, for instance, has a noticeably eccentric orbit compared to Earth. This means that when Mars is at opposition and also near its perihelion, it's significantly closer to Earth than at other times. Hence, its angular diameter will appear much larger. Conversely, Jupiter's orbit has low eccentricity, meaning its distance from Earth doesn't vary as much, leading to the less variable angular diameter. By understanding orbital eccentricity, students can appreciate the dynamic nature of celestial mechanics and better predict the changing features of planets in the night sky.