Space Physics: The Earth, the Solar System, Stars and the Expanding Universe
Cosmic Navigator: From Day and Night to the Big Bang 🌌
Introduction
1. Introduction
Sunrise in the east, a full Moon that was a thin crescent last week, a night sky dusted with stars that are really other suns: space physics is the chapter where everyday sights turn into a tour of the whole Universe. This is your five-minute refresh on why we get day and night and the seasons, what lives in the Solar System and how gravity keeps it in orbit, what the Sun and stars are made of, and how redshift tells us the Universe is still expanding. Every idea below is one mark-earning sentence away from the exam. Let's go cosmic! 🌌
2. Spinning Earth, Orbiting Moon
The Earth spins once on its axis every 24 hours, and that single spin gives us day and night: the half facing the Sun has day, the half turned away has night. Because the Earth turns from west to east, the Sun appears to rise in the east and set in the west. The Earth also orbits the Sun once every year (about 365 days), and its axis is tilted by about . The axis keeps pointing the same way in space, so for part of the orbit the northern hemisphere leans toward the Sun and half a year later it leans away. The hemisphere tilted toward the Sun gets more direct sunlight and more hours of daylight, so it has summer while the other hemisphere has winter. (It is not because the Earth is closer to the Sun: the two hemispheres have opposite seasons at the same time.) The Moon orbits the Earth about once a month. As it orbits, we see different lit fractions of it, which is why it cycles through its phases from new to full and back.
Key idea🔑 Key idea: Earth spins once per 24 h → day and night. Earth orbits the Sun on a tilted axis → 1 year and the seasons. Moon orbits the Earth → about 1 month and the phases.
Worked example
Worked Example: Counting Days in a Year
Worked Example: How Many Spins per Lap? 🔄
The Earth spins once every 24 hours and takes about 365 days to orbit the Sun once. Roughly how many complete spins does the Earth make during one orbit?
- 1One spin takes one day, so the number of spins equals the number of days in the orbit: about 365 spins per year. That is why a "year" and a "day" measure two completely different motions, the orbit and the spin.
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3. A Star, Eight Planets and a Lot of Gravity
The Solar System is one star (the Sun) plus eight planets, minor planets (dwarf planets such as Pluto, and asteroids in the belt between Mars and Jupiter), moons (natural satellites) and comets, all formed long ago from a cloud of gas and dust pulled together by gravity (the accretion model). The four inner planets (Mercury, Venus, Earth, Mars) are small and rocky; the four outer ones (Jupiter, Saturn, Uranus, Neptune) are large and gaseous. The Sun contains most of the mass of the Solar System (over 99%), so its gravitational attraction is what keeps the planets and everything else in orbit. A planet's surface gravitational field strength depends on the planet's mass, and the field gets weaker further from the planet. Comets are small icy bodies on long, stretched orbits; near the Sun their ices stream away as a tail. Distances are so big that light takes minutes to cross them: , and sunlight takes about 8 minutes to reach us.
Key idea🔑 Key idea: The Sun has most of the Solar System's mass, so its gravitational attraction keeps the planets in orbit. Inner planets rocky and small, outer planets gaseous and large.
Worked example
Worked Example: A Radio Command to a Probe
Worked Example: Four Minutes to Mars 🐌
A control centre sends a radio signal to a probe m away, near Mars. Radio waves travel at m/s. Calculate the time the signal takes to reach the probe.
- 1Rearrange distance = speed × time for the time: .
- 2Divide the front numbers and subtract the powers of ten: and , so s, about minutes each way. That is why rovers are given goals to drive to rather than being steered live.
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4. The Sun, the Stars and the Milky Way
The Sun is a medium-sized star made mostly of hydrogen and helium. It radiates most of its energy as infrared (the part you feel), visible light (the part you see) and ultraviolet (the part that burns). The Sun is just one of billions of stars in our galaxy, the Milky Way, which is itself one of billions of galaxies in the Universe. Space is so vast that we measure it in light-years: a light-year is the distance light travels through space in one year, not a time. You can turn it into metres with distance = speed × time (see the worked example below). Light from the Sun takes 8 minutes to reach us; light from the next nearest star takes over 4 years. 🌟
Key idea🔑 Key idea: A light-year is the distance light travels in one year. The Sun is a medium-sized star, mostly hydrogen and helium, radiating mainly infrared, visible and ultraviolet.
Worked example
Worked Example: How Far Is a Light-Year?
Worked Example: Light's One-Year Road Trip 🛣️
Light travels at m/s. One year is about seconds. Show that one light-year is roughly m.
- 1A light-year is just a distance, and distance = speed × time. Write the equation with the right values:
- 2Multiply the numbers and add the powers of ten:
- 3So one light-year m, about 9 and a half thousand million million metres. That is why even the nearest star, just over 4 light-years away, is impossibly far to ever reach by rocket: the number is set by light's speed, the fastest thing there is.
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5. Redshift and the Expanding Universe
When we split the light from a distant galaxy into its spectrum, the pattern of lines is shifted toward the red (longer-wavelength) end. This redshift is the fingerprint of a galaxy moving away from us. The clincher is that the further away a galaxy is, the bigger its redshift, so the most distant galaxies are racing away the fastest. The only sensible explanation is that the whole Universe is expanding, carrying the galaxies apart. Rewind that expansion and everything was once packed together, which is exactly the picture of the Big Bang: the Universe began from a single hot, dense point and has been expanding ever since. Redshift is the key evidence for it. 💥
Key idea🔑 Key idea: Redshift means galaxies are moving away; further galaxies show more redshift, so the Universe is expanding. This is the main evidence for the Big Bang.
Worked example
Worked Example: Reading Two Redshifts
Worked Example: Near or Far? 🔭
Galaxy A's light shows a small redshift; galaxy B's light shows a much larger redshift. Which galaxy is further away, and which is moving away faster?
- 1Bigger redshift means both further away and moving away faster, so galaxy B is the more distant one and the faster-receding one. This "further = faster" pattern across all galaxies is exactly what tells us the Universe is expanding.
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