Mars stops the show – Part 2
An elliptical orbit is egg-shaped, with a long axis and a shorter one. As planets orbit the “egg’s” centre, their distances from it, and from each other, vary—according to where they are, at a given time.
Be sure to picture this egg-like configuration clearly. It’ll enable you to visualize the “markedly elliptical” orbit of Mars, which is the attribute that causes changes in its visibility.
Technically, astronomers ascribe the increasing brightness of Mars, to three aspects of its orbital dynamics: (a) “Opposition”; (b) “perihelion”; and (c) its “closest approach” to Earth.
But first, when a planet reaches the end of the ellipse that is farthest off-centre, astronomers say it is at “aphelion”—its greatest distance from the Sun. Conversely, they call the closest point “Perihelion”.
Mars is one of the five “superior” planets, which orbit the Sun at a greater distance than Earth. Since closer planets move faster, Earth passes between Mars and the Sun periodically.
This passage occurs every 26 months or so, and is termed “opposition”—because, from our vantage point, the Red Planet then lies directly opposite the Sun. In 2016, opposition falls on May 22nd.
By way of illustration, a “full Moon” is really a lunar opposition. The Moon appears brighter than at other times, because we are positioned between it and the Sun, facing its illuminated half squarely.
The apparent brightness of a reflective satellite, at opposition, depends to a very large extent on its orbital position.
With the lunar orbit being elliptical, for example, some full Moons are brighter than others—because the distance between Earth and its satellite is not always the same, at opposition.
In the case of Mars, its proximity to the Sun and to the Earth are both important parameters. The closer Mars is to the Sun and to Earth, at opposition, the brighter it will appear in the night sky.
On June 14th, the Sun separated Mars and Earth. A straight line extending from Earth, and passing through the Sun, would have connected with the Red Planet on the other side.
During “conjunction,” as this alignment is called, Mars was hidden from naked eye view; and telescopic observation was unsafe, due to the Sun’s blinding radiance.
But in July, it emerged as an apparition. “As a general rule,” explains Mars specialist, Jeffrey D. Beish, “an ‘apparition’ begins when a planet emerges from the glare of the Sun shortly after conjunction”.
Mars is now swinging in closer to Earth. At the same time, it is also looping Sunward, in its elliptical orbit—and becoming more luminous, in the process.
Bruce McClure of Earth-Sky.Org, writes: “Oppositions that closely coincide with Mars at perihelion are much brighter and spectacular than [those] that happen with Mars swinging out to aphelion”.
He cites, as a reference standard, the historic Mars opposition of 2003—when the planet was closer to the Earth and to the Sun than it had been in 60,000 years!
This year, we’ll pass between Mars and the Sun on May 22nd and the planet with make its closest approach to Earth, eight days later. But perihelion is the odd-man-out, arriving tardily October 29th.
It’ll be 284 years, McClure notes, before the planet appears as bright in the sky, as it did during the 2003 opposition.
Even so, Mars and its glittering retinue is still a show that is well worth watching. But if you insist on being finicky…
Well, just exercise some patience. Astronomers predict an even more spectacular display, than the opposition of 2003. It’ll happen August 29, 2287.