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School Physics Notes: Astronomy 2. Model of our Solar System and beyond

ASTRONOMY: 2. Our contemporary model of our Solar System - a sun, planets, their moons and beyond

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INDEX physics notes on ASTRONOMY

See also notes on the geology and atmosphere of our moon and the planets of our solar system


2. Our contemporary model of our Solar System and beyond

The 8 major planets, minor planets and asteroids orbit the Sun in slightly elliptical orbits (our 'Solar System'), but our Sun is just one of millions-billions of stars in our galaxy (we see part of it as the 'Milky Way') and in turn the observable universe itself contains billions of other galaxies.

(a) Our Solar System - the Sun (a main sequence star) and 8 orbiting major planets, and minor planets and asteroids.

The movement of the planets and asteroids has been observed from visible light (reflected sunlight) for thousands of years, initially with the naked eye and from the early 16th century onwards, with telescopes.

8 major PLANETS Distance from Sun in Mkm Mass relative to Earth Surface gravitational field constant in   m/s2 or N/kg Size relative to Earth Time to orbit Sun (days or years) Axis rotation time Average surface temperature oC
Mercury 58 0.05 3.7 0.4 88 d 58.6 d +350
Venus 108 0.8 8.8 0.9 225 d 242 d +480
Earth 150 1 9.8 1 365 d 24 h +22
Mars 228 0.1 3.8 0.5 687 d 24.7 h -23
Jupiter 778 318 25 11 12 y 9.8 h -153
Saturn 1430 95 10.5 9.4 29 y 10.8 h -185
Uranus 2870 15 10.4 4 84 y 17.3 h -214
Neptune 4500 17 12.8 3.8 165 y 16 h -225
Pluto (dwarf planet) 5915 0.003 0.49 0.2 248 y 153 h -236

The force of gravity

Generally speaking the gravitational field constant at the surface of a planet increases with its mass.

The gravitational constant of our Moon is 1.7 m/s2 or 1.7 N/kg.

The gravitational constant of our Sun is 293 m/s2 or 293 N/kg.

The strength of the gravitational pull decreases the further you are from the centre of the planet (the attractive force decreases according to an inverse square law: force 1 / distance2)

With modern techniques, the Sun, at the centre of our Solar System, can be observed by detecting emissions in various regions of the electromagnetic spectrum eg infrared, visible light, ultraviolet, X-rays and even gamma ray emissions.

Orbital paths

The orbits are not quite perfect circles, but slightly 'squashed' into an elliptical shape.

The Earth is the 3rd planet from the Sun - see above data table on the planets.

The Sun is ~150 million km away from us and sunlight takes ~8 minutes to reach us.

Mercury, Venus, Earth and Mars are the four inner planets, relatively small and consisting mainly of rock.

The gas giant planets Jupiter, Saturn, Neptune and Uranus have gases such as hydrogen, ammonia, methane and carbon dioxide their atmosphere, may have rocky cores?

(b) Our Milky Way - is our view of looking through our own galaxy

The name 'Milky Way' comes from the profusion of bright starlight from our galaxy when you look through its centre against the background of the relatively dark night sky.

Our solar system is just one small part of a galaxy - which is a massive collection of billions of stars held together by gravity.

The Milky Way rotates around the central core of the galaxy and astronomers think there is a massive black hole there.

Until relatively recently, the Milky Way galaxy, has been observed with the naked eye and then telescopes on Earth, but now it can be viewed through powerful telescopes on satellites eg the Hubble Space Telescope.

Our galaxy, and for that matter distant galaxies, can be continually observed using everything from giant radio telescopes, huge optical\visible light telescopes to gamma ray burst detectors.

(c) 'Outer space' and nebulae

Beyond the Earth and beyond our own solar system and galaxy, it is far from being a vacuum of 'emptiness'.

Interstellar space (between stars) contains huge clouds of dust and a mixture of gases, mainly hydrogen and helium gases, but traces of lots of other molecules including organic molecules.

These clouds are where stars are formed and are called nebulae. A nebula is an enormous cloud of dust and gas occupying the space between stars and acting as a nursery for new stars.

(d) The Universe is everything - see separate page Cosmology - the Big Bang Theory of the Universe

The cosmos is a term used to describe the universe seen as a well-ordered whole.

A mathematics note on distances - a sort of perspective on 'everything'!

The distance from planet Earth to the Sun is 150 million kilometres.

150 Mkm, 150 000 000 km, and in standard form 1.5 x 108 km.

The distance from the Sun to the dwarf planet Pluto is 5915 million km.

5915 Mkm, 5 915 000 000 km, in standard form ~5.9 x 109 km.

The diameter of our galaxy, the 'Milky Way', ~1 000 000 000 000 000 000 km, in standard form ~1.0 x 1018 km.

Some derived calculations, taking the speed of light to be 3.0 x 108 m/s.

Ex. 1. How long does it take light to travel from the Sun's surface to the Earth?

speed = distance / time,   time = distance / speed

time = 1.5 x 108 x 1000 / 3 x 108 = 500 s, 8 minutes and 20 seconds.

Ex. 2. How long does it take light to cross from one side of our galaxy to the other?

time = (1018 x 1000) / 3 x 108 = ~3.33 x 1012 s

1 Earth year is 365.25 x 24 x 60 x 60 = 31557600 s

time = 3.33 x 1012 / 31557600 = ~106 000 years!


See also notes on the geology and atmosphere of our moon and the planets of our solar system

INDEX of my physics notes on ASTRONOMY


Keywords, phrases and learning objectives for astronomy

Be able to describe the contemporary model of our Solar System of our sun, planets and moons.

Beyond our solar system be able to describe what the following are: stars, galaxies and the universe.


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