The Major Planets
Celestial Magnitudes - A Convenient
Misconception
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| Our Solar System? |
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This section begins with a confession: almost every depiction of the celestial motion and the positions of celestial bodies presented thus far has been misleading. (This includes the illustration on the right.) However, the misconceptions perpetuated are not unique to this website’s discussion of the basics of celestial motion. These are misconceptions perpetuated by nearly every astronomy textbook ever written and by every astronomy professor who has ever drawn on a blackboard, and they are summed up by the key disclaimer: "not drawn to scale".
The Ordinal Solar System
Reference:
[Wikipedia, "Planet"]
In modern astronomy when our solar system is discussed it is typically characterized in an ordinal fashion in terms of the major planets and their successively increasing distances from the Sun. The four planets orbiting closest to the Sun are Mercury, Venus, Earth and Mars, also known as the terrestrial planets, which are similar in size and are composed primarily of rock. The four remaining planets, Jupiter, Saturn, Uranus and Neptune are considerably more massive than the terrestrials and are composed primarily of gaseous materials, with Uranus and Neptune also being composed of rock and ice. Between Mars and Jupiter lies the Asteroid belt which is composed of tens of thousands of rocky celestial bodies or “minor planets” called asteroids which also orbit the Sun. Rounding out this general characterization of the solar system is the planet Pluto which was recently demoted from major to minor planet status. Pluto is often included in a group of lesser known celestial bodies called the trans-Neptunian objects.
Putting it All on One Page
When these celestial bodies are depicted together it is frequently with their relative sizes and distances distorted so as to combine all of the celestial bodies onto a single image or page. The cartoon image in the upper right hand corner is typical of these convenient characterizations, where the order of the planets is preserved and their relative distances and sizes greatly diminished to fit every object into a single picture. The first image below depicts the eight major planets and the Sun with their relative sizes shown to scale. This image really demonstrates why Jupiter and Saturn are often called the “gas giants” of the solar system. (Also shown are the large asteroid Ceres, the minor planet Pluto and another trans-Neptunian object called Eris.)
| Major Planets and Sun Shown to Scale |
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| Image courtesy of NASA and Wikipedia |
The second illustration below depicts the true relative distances from the Sun of each planet as well as asteroids in the Asteroid belt, measured in astronomical units or (AU) which is equal to the length of the semi-major axis of the Earth's elliptical orbit or 149,597,870.69 km. Notice that the Earth is positioned at 1 AU and the farthest major planet Neptune is positioned at 30 AU. Also notice that Pluto's distance from the Sun varies between 30 and 50 AU, with its orbit sometimes crossing that of Neptune.
| Relative Distances of Celestial Objects from Sun |
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| Image courtesy of Greg Goebel's IN THE PUBLIC DOMAIN |
Earth as the Standard Bearer in the Solar System
Reference: [GSU HyperPhysics]The solar system scale diagram illustrates an important convention used in modern astronomy when characterizing celestial bodies in the solar system. This convention uses the Earth's size, mass and distance from the Sun as a standard unit with which to characterize every other object. For example, the Earth's equatorial diameter is 12,756 km and its mass is 5.976 x 1024 kilograms, and Venus which is considered to be the most similar in size to the Earth has a diameter of 12,104 km or 0.949 Earth diameters, and a mass of 4.87 x 1024 kilograms or 0.815 Earth masses. The table below summarizes a few of the key characteristics of each planet relative to the Earth's standard distance from the Sun, its diameter and mass.
| Characteristics of Planets Relative to Earth |
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| Data Extracted from Georgia State University HyperPhysics |
Apparent or Angular Sizes of Celestial Bodies
One interesting fact that can be culled from these planetary data is the apparent or angular size of each of the planets as they appear to an observer from Earth. In the discussion on solar eclipses in the website section on The Earth and the Moon, the moon's angular size, which averages about 0.52 degree, is shown to match closely the angular size of the Sun at 0.53 degree, explaining why the Moon is able to block the Sun's rays during a solar eclipse. A celestial body's angular or apparent size is defined in terms of its actual size or diameter and its distance from the Earth according to
The table below lists the diameter and average distance from the Earth for the Sun, Moon, the eight major planets and Pluto. Five planets are visible to the naked eye, Mercury, Venus, Mars, Jupiter and Saturn and are considered the "classical" planets observable by ancient astronomers. Notice that the apparent size and visiblity of the planets is dependent on both the planet's size and its distance from the Earth. For example, although Mars is more than 10 times closer to the Earth than Jupiter, Mars appears to be only half the size of Jupiter, 0.00695 degree vs. 0.1395 degree, because of the massive size of Jupiter. Similarly, while Saturn is 13 times further from the Earth than Mercury, the ringed planet's apparent size is greater than that of Mercury, 0.00574 degree vs. 0.00305 degree.
| Apparent or Angular Sizes of Sun, Moon and Planets "Classic Planets" Visible to Naked Eye |
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| Data Extracted from GSU HyperPhysics and FactMonster.com |
The next section recommends a number of online resources that can be used for further study of the relative motion and positions of the major planets as well as other celestial bodies like asteroids, comets and meteors.
| The Major Planets - A Convenient Misconception | Online Planetary Astronomy Resources |
Basics of Celestial Motion. Copyright 2006 S. E. Scruggs
