Hunters of planets

Our ancestors did a lot of sky watching. They did not understand much of what they were seeing, but they were good observers.

They used the sky as a clock and calendar. Today, apart from the weather, we live largely independent of the sky.

Those early astronomers noticed the Sun, Moon and stars move from east to west, and that the stars form unchanging patterns, constellations, which got named mostly after characters, animals and objects in myth and legend.

However, there were some star-like objects that moved against the stars. Unlike the stars, they shone steadily, like lamps, and because of their changing positions, they were called wandering stars, or planets.

Those ancient observers also noted that the Sun, Moon and planets always wandered within a defined strip of sky, which we now call the ecliptic.

This path passes through 13 constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius and Pisces.

These are the constellations of the Zodiac. Having 13 signs of the zodiac was not popular, so Ophiuchus was quietly dropped. We see all these bodies moving along one strip in the sky because all the planets orbit the Sun in the same plane, like marbles rolling around a plate.

This is useful in that to find planets we just search along the ecliptic.

Our ancestors spotted five wandering stars, which came to be named, Mercury, Venus, Mars, Jupiter and Saturn.

These planets are bright and easy to spot with the unaided eye; in fact, Venus and Jupiter can be so bright they are impossible to miss.

Further discoveries had to wait for the development of the telescope.

There is a rather strange thing called the Titius-Bode Law, in which a totally arbitrary (as far as we know) formula that predicts, remarkably accurately, the distances from the Sun at which planets can be found.

The formula gave distances at which planets might be found beyond Saturn, and in 1781, William Herschel started looking. On May 13, he discovered the planet Uranus. From here on, calculation and position measurement became the main tools in finding new planets.

Planets orbit the Sun because they attract each other gravitationally. It is easy to predict the positions of a single planet orbiting the Sun.

However, when there are multiple planets, they tug at each other, making tiny distortions in each other's orbits. This provides a powerful method for finding unknown planets.

We calculate the positions of known planets taking into account the perturbations by the other known planets. If we find a discrepancy, we can calculate how big a body is needed to explain that discrepancy, and also where to look for it.

From differences between the predicted and observed positions of Uranus, John Couch Adams and Urbain Le Verrier independently predicted the position of the new planet. Adams passed the prediction to the U.K. Astronomer Royal, George Airy, who conducted an unsuccessful search.

Le Verrier persuaded Johann Gottfried Galle to look for the new planet, and on Sept. 23, 1846, Galle found it. The new planet was named Neptune, after the God of the Sea, because of its deep, blue colour.

Perturbations in Neptune's orbit led to the prediction of yet another planet, along with a rough idea as to where to look.

On Feb. 18, 1930, Clyde Tombaugh found it. The new world was named Pluto. However, its mass was too small to explain those perturbations.

We now know that Pluto is just one of a very large number of similar bodies orbiting in the outer Solar System, all contributing to the distortion of Neptune's orbit.

We cannot say if there are any more planets out there.

  • Mars is low in the southwest after dark.
  • Jupiter and Saturn lie low in the southeast before dawn.
  • The Moon will reach First Quarter on the 19th.

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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