Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 6: SPACE PHYSICS
Have you ever wondered about the vastness of space? How many stars might exist in the universe? Scientists believe that while the universe has an observable size limit - one that is difficult to comprehend - it is astonishingly immense.
One of the best ways to visualize this is through an animation that takes you on a journey from Earth to the farthest reaches of what we can observe. This captivating video not only provides perspective but also introduces several key concepts:
YouTube video: The size of the universe
Cosmic Voyage - The US National Air and Space Museum
In this topic on Space Physics, we will learn about all astronomical objects, from the Earth out to the edge of the known Universe.
Much of this section on the Earth (and Moon) is about ideas you may well know already from previous learning. However it is still part of this IGCSE course so we will recap how the Earth and Moon move, giving rise to day and night, phases of the Moon, as well as the four seasons.
The Earth rotates on its axis, as shown in Figure 1 below. The Sun, which is 150 million kilometres away, only shines on one half of the Earth as shown. This means that countries move from sunlight into night time as the Earth rotates.
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Figure 1: The rotating Earth
Notice that one side is in sunlight!
Marvel CC BY-SA 3.0
As the Earth takes 24 hours to rotate, the side facing the Sun should have 12 hours of daylight. However this time period is complicated by the tilt of the Earth and the seasons (see below). For example in summer months, countries away from the equator have longer days and shorter nights.
From our perspective, it is the Sun that moves across the sky, but it is actually the Earth rotating that gives rise to day and night. In figure 1 above, you can see that the Earth spins 'Eastward' - and this means that from our point of view, the Sun rises in the East, and 'sets' (drops below the horizon) in the West.
The Earth rotates around an axis that passes from the North to South poles. However did you know that this Earth is titled? Scientists think this tilt may have come from a collision with another minor planet early in the Earth's formation. The tilt, which is about 23 degrees, gives rise to some interesting effects:
It takes just over 365 days for the Earth to orbit the Sun. As it does this, different hemispheres of the Earth are 'tilted' towards or away from the Sun as shown in Figure 2:
Figure 2: The Tilted Earth Orbiting the Sun
Tauʻolunga, via Wikimedia Commons
On June 21st, the northern hemisphere is tilted towards the Sun as shown in figure 2, with the Southern hemisphere tilted away. This leads to summer in the north and winter in the south.
Conversely, on Dec 21st, it is summer in the south and winter in the north as the tilt of the Earth means the southern hemisphere receives more sunlight. This is shown in more detail in figure 3:
Figure 3: The Tilted Earth causing summer in the south
Note: The orbit of the Earth is not quite a circle, it is 'elliptical' (see below for more on orbits). However the variation in distance from the Sun to the Earth is very minor and this is not the cause of seasons!
The Moon is the Earth's only natural satellite, and is very large compared to the Earth. Other planets tend to have moons that are much smaller than the parent planet. The moon takes about 28 days to orbit the Earth. As the Moon orbits the Earth, the sunlight only lights up the half facing the Sun. From the Earth, this means we see varying levels of light on the moon, leading to what are known as the Moon's phases. These are shown in figure 4 below.
Figure 4: The Moon's Phases explained
Andonee CC BY-SA 4.0
Note: You are not expected to learn the names of the phases!
Figure 5: Time Lapse of the Phases of the Moon
Tomruen, Public domain, via Wikimedia Commons
If you are interested, the phases of the Moon take 29.5 days to repeat, not 27! This is because in 27 days, the Earth has moved in orbit around the Sun, and it takes an additional 2.5 days before the Sun, Earth and Moon are in the same relative positions. The Moon has to travel a little further than one full orbit to reach the same point relative to the Sun.
Questions:
1. A student writes down the time periods for the Earth and Moon as follows:
Tx = time for Earth to orbit the Sun
Ty = time for Moon to Orbit the Earth
Tz = time for Earth to spin on it's axis
Which one of the following lists these time periods from shortest time to longest time?
The answer is D.
Tx = time for Earth to orbit the Sun is 365 days,
Ty = time for Moon to Orbit the Earth is 27 days,
Tz = time for Earth to spin on it's axis is 24 hrs.
2. Explain how the tilt of the Earth's axis gives rise to seasons.
In this answer, you should write a paragraph, perhaps with a diagram to help, similar to the diagram in figure 2 or 3. This will probably be a question where a wide range of statements are acceptable. Ideally your answer should include:
Moons generally travel around planets in a near circular orbit. This means that they keep approximately the same distance from the planet at all times. This is also true of planets as they orbit the Sun. The Earth stays at approximately the same distance from the Sun all year as it travels around in its orbit. However mathematically speaking, all astronomical orbits are actually ellipses, not true circles.
Figure 6 below show five imaginary planets orbiting a star, all with varying orbits. The red planet is a circular orbit, and the other 4 showing more 'squashed' elliptical shapes. Mathematicians measure this using 'eccentricity' (not on the syllabus). The purple orbit (furthest right) is the most 'eccentric' orbit, showing the shape of a very elliptical orbit.
Figure 6: Elliptical Orbits explained
Phoenix7777 CC BY-SA 4.0
The elliptical orbits of planets and other objects in the Solar System will be discussed in the next section.
The maths for elliptical orbits is complicated and is not covered in this course. However, for the near circular orbits of moons and planets it is pretty straightforward:
| average speed = | distance |
| time |
Therefore we can replace the distance and the time in this formula to give us the speed around a circular orbit:
| average orbital speed = | 2 x π x orbital radius |
| time period |
| v = | 2 x π x r |
| T |
The speed is the average speed because, as discussed above, true orbits are ellipses and the orbital speed will change a little during the orbit. Note that there are no units given here and you will need to use some common sense. If a question gives the radius in kilometres (km) and the time in hours (h), then the speed will be in units of km/h.
Have a go at these questions to test your understanding of this formula, and the rest of this section:
Questions:
3. The Moon orbits the Earth in approximately 708 hours, with a radius of orbit of 385 000 km. Using the formula given above, calculate the average orbital velocity of the Moon.
Using the formula
| v = | 2 x π x r |
| T |
| v = | 2 x π x 385 000 |
| 708 |
4. The Hubble Space Telescope (HST) orbits the Earth at a speed of 7.6 km/s and has an orbital time period of 5700 seconds.
a) Both time period and speed include seconds, so we can substitute the numbers directly into the formula:
| v = | 2 x π x r |
| T |
| 7.6 = | 2 x π x r |
| 5700 |
| r = | 7.6 x 5700 |
| 2 x π |
b) The Earth's radius is 6400 km, so the HST's height above the Earth's surface is 6900 - 6400;
so the distance = 500 km
More questions on this topic follow the next section, 6.1b on the Solar System.
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