# Astronomy related laws presented with relevant animations

Astronomy is a natural science which is the study of celestial objects. It encompasses the study of evolution of such objects, and phenomena that originate outside the atmosphere of Earth, including supernovae explosions, gamma ray bursts, and cosmic microwave background radiation. The feature covers some astronomical laws with their respective animations.

### Kepler’s First Law of Planetary Motion Image Courtesy: physicsclassroom.com

Kepler’s first law is sometimes referred to as the law of ellipses. The law explains that planets are orbiting the sun in a path described as an ellipse with the Sun at one focus of the ellipse. The planet, as seen in the animation, follows the ellipse in its orbit, which means that the Earth-Sun distance is constantly changing as the planet goes around its orbit. The animation depicts the elliptical orbit of a planet around the sun.

### Kepler’s Second Law of Planetary Motion The second law states that the line joining the Sun and a planet sweeps through equal areas in an equal amount of time. An implication of Kepler’s second law is that a planet moves faster when it is closer to the Sun and slower when it is more distant.

### Kepler’s Third Law of Planetary Motion Image Courtesy: pd.astro.it

Kepler’s third law quantifies the observation that more distant orbits have longer periods.  Unlike Kepler’s first and second laws that describe the motion characteristics of a single planet, the third law makes a comparison between the motion characteristics of different planets. The comparison being made is that the ratio of the squares of the periods to the cubes of their average distances from the sun is the same for every one of the planets. The animation shows two planets orbiting the Sun but one orbit is 1.5874 larger than the other. Notice that the bigger orbit takes twice as long to go around the Sun. This is due to

R13/R23 = 1.58743 = 4 = 22 = P12/P22

as required by Kepler’s Third Law.

### Titius- Bode’s Law Image Courtesy: splung.com

Titius- Bode law is an empirical law that appears to calculate the radii of the orbits of the planets. According to this law, distance ‘r’, of the nth planet from the Sun (in A.U.s) is given by:

rn = 0.4 + 0.3 x 2n

Take the number sequence 0.4 + [0.3 x (2n)]. The results are: .4, .7, 1.0, 1.6, 2.8, 5.2, 10.0 and 19.6. The results present within a few percent the average distance in astronomical units (AU) of each Planet from the Sun. The exception is at 2.8 AU, where we find the asteroid belt exactly where Bode’s Law predicted a planet. The asteroid belt (as shown in the animation) is thought to be the remnants of a planet that did not quite make it because of the perturbing influence of the gravity of Jupiter. Why does Bode’s Law work? Many scientists believe that it is just a coincidence and there is no comprehensive theory but part of the answer could be planetary resonances. This is where the orbital period of the planets is some integer of another planet which has the effect of perturbing the planets to stable orbits. Bode’s Law does not work for the orbits of Neptune. The Titius-Bode law was used to help discover Ceres, a 1000 km asteroid, in 1802, and Uranus in 1781.

### Hubble’s Law Image Courtesy: e-education.psu.edu Image Courtesy: universe-review.ca

This law describes a relationship between a galaxy’s distance from us and its velocity through space. The farther away a galaxy is from us, the faster it is receding from us. The numerical form is:

V = Hd

Where V is the speed at which a distant object is receding from us, d is its distance, and H is the Hubble constant. Today, the velocity is estimated to be 71 kilometers per second per megaparsec, plus or minus 7; which is about 21 km/sec per million light-years. This means that an object a million light-years away would be receding from us at 21 km/sec; an object 10 million light-years away, 210 km/sec and so on. The animation is that of a loaf of raisin bread dough which rises during baking and all of the raisins move further away as well. Let’s say that the size of the dough doubles so the distance between all of the raisins will double and in turn the more distant raisins will appear to have moved faster. Hubble’s Law is interpreted as evidence that the Universe is expanding.

### Kirchhoff’s Laws

Objects with different temperatures and compositions emit different types of spectra. By observing an object’s spectrum, then, astronomers can deduce its temperature, composition and physical conditions, among other things. Kirchhoff’s laws help the astronomers further in their observations.

Image Courtesy: home.snc.edu

First Law:

A hot solid, liquid or gas, under high pressure, gives off a continuous spectrum.

Second Law:

A hot gas under low pressure produces a bright-line or emission line spectrum.

Third Law:

A dark line or absorption line spectrum is seen when a source of a continuous spectrum is viewed behind a cool gas under pressure.

The wavelength of emission or absorption lines depends on what atoms or molecules are found in the object under study.

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