0625 · 6.1Earth and the Solar System

Earth, seasons and Moon phases

0:00The daily movement of the Sun, the changing seasons and the Moon’s phases all follow repeating patterns. Each pattern comes from motion in space, but the motions are different. Earth spins and circles the Sun, while the Moon orbits our planet. Understanding these movements connects familiar observations to the structure of the Solar System. We will first explain the patterns we observe from Earth. Then we will explore the Solar System’s objects, their formation and the gravity controlling their orbits.

0:27We will also calculate orbital speeds and light travel times. Throughout the video, we will connect each calculation to what is physically happening. Three ideas will help us begin. Rotation means spinning about an axis, which is an imaginary line through an object. An orbit is the path one object follows around another. Gravity is an attractive force between masses, and it acts even when the objects are not touching.

0:50Earth is a planet that rotates on its tilted axis once in approximately twenty-four hours. At any moment, the side facing the Sun receives sunlight and experiences daytime. The side facing away experiences night. As Earth rotates, a particular place moves into sunlight and then into darkness. Earth rotates from west to east, making the Sun appear to move across our sky in the opposite direction. We therefore observe the Sun generally rising in the east and setting in the west.

1:15This daily motion is apparent rather than the Sun orbiting Earth each day. Our changing viewpoint produces the movement we see. Earth also orbits the Sun once in approximately three hundred and sixty-five days. Its axis remains tilted in approximately the same direction throughout this journey. Consequently, the northern hemisphere leans towards the Sun during one part of the orbit and away during another. This repeating arrangement produces the yearly cycle of seasons.

1:42When a hemisphere tilts towards the Sun, it receives more direct sunlight and has longer daylight hours. More direct sunlight concentrates the incoming energy over a smaller area. Longer days also allow heating to continue for more time. Together, these effects produce summer rather than winter. When that hemisphere tilts away, sunlight arrives at a shallower angle and spreads over a larger area. Daylight hours are also shorter, so less energy arrives each day.

2:07Meanwhile, the other hemisphere experiences the opposite season. Seasons therefore result from Earth’s tilted axis combined with its yearly orbit, not from changing distance from the Sun. The Moon takes approximately one month to orbit Earth. It does not produce its own visible light, but reflects sunlight. The Sun always illuminates approximately half of the Moon. As the Moon travels around Earth, we see different amounts of that illuminated half.

2:32Near new Moon, the illuminated half mostly faces away from Earth. Near full Moon, the illuminated half faces towards us. Between these positions, we see crescents, half moons and larger illuminated shapes. These are phases, and their sequence repeats approximately monthly as the Moon continues its orbit. Ordinary Moon phases are not caused by Earth’s shadow. They depend on our viewing angle towards the Moon’s sunlit half. Earth’s shadow can darken the Moon during a lunar eclipse, but that is a different event.

3:01For this topic, keep the monthly phase cycle linked to the Moon’s changing position around Earth.

Solar System objects and formation

3:07The Solar System contains one star, the Sun, and eight planets. Their order from the Sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four nearest planets are relatively small and rocky. The four furthest planets are relatively large and gaseous by comparison. That comparison describes their broad structures, not an absence of solid material inside the outer planets. Jupiter and Saturn contain large amounts of gas.

3:33Uranus and Neptune also contain substantial amounts of other materials beneath their atmospheres. For this syllabus, the essential contrast is small and rocky inside, large and gaseous outside. Other Solar System objects include minor planets orbiting the Sun. These include dwarf planets such as Pluto and asteroids, many of which occupy the asteroid belt between Mars and Jupiter. Moons orbit planets and are natural satellites. A natural satellite is a naturally occurring object orbiting another larger body.

4:02Comets are smaller Solar System bodies containing ice, dust and rocky material. They also orbit the Sun, often along very elongated paths. When a comet approaches the Sun, heating can release material that forms a visible tail. Moons and natural satellites are overlapping descriptions, rather than entirely separate categories. The accretion model explains how the Solar System formed from an interstellar cloud of gas and dust. Interstellar means between stars, and this cloud contained many different elements.

4:31Gravity pulled its material together, making denser regions attract more matter. The cloud was rotating, and the collapsing material formed a flattened accretion disc. Most material gathered near the centre and eventually formed the Sun. Within the surrounding disc, small particles collided and stuck together. Larger clumps attracted additional material through gravity and gradually grew into planets. Accretion means growth through the accumulation of matter, so gravity was central to this process.

4:57Near the developing Sun, temperatures were high, so materials such as rock and metal could remain as solid particles. Many other substances stayed gaseous there. Further away, cooler conditions allowed more materials, including ices, to collect into growing bodies. Some outer bodies became massive enough to attract large amounts of surrounding gas. This explains the broad difference between the inner and outer planets. The inner planets developed mainly from rocky and metallic material and remained relatively small.

5:24The outer planets could accumulate more material and substantial gaseous envelopes. Their different formation conditions produced different sizes and compositions, rather than identical planets at different distances.

Gravity and elliptical orbits

5:35The Sun contains most of the mass of the Solar System. Its gravitational attraction therefore dominates the motion of the planets and keeps them in orbit. Without that attraction, a planet would continue along a straight path rather than curve around the Sun. Gravity continually changes the direction of the planet’s motion. An orbiting planet is not outside gravity, and its motion does not require a forward push from the Sun.

5:57The gravitational force acts towards the Sun while the planet has motion across that direction. Combining these produces a curved path. Gravity supplies the force needed to keep the object in orbit. Gravitational field strength describes force per unit mass and is measured in newtons per kilogram. At a planet’s surface, it depends on the mass, although the radius matters too. For planets of the same radius, greater mass produces a stronger surface gravitational field.

6:23Moving further away from a particular planet makes its field weaker. The following orbital ideas are Supplement content for Extended students. Planets, minor planets and comets have elliptical orbits. An ellipse is a shape like a stretched circle, although some ellipses are almost circular. The Sun lies at one focus rather than the geometric centre, unless the orbit is nearly circular. An elliptical orbit therefore carries an object through changing distances from the Sun.

6:50The Sun’s gravitational field becomes weaker as distance from it increases. Comparing the planets, those further from the Sun also have lower orbital speeds. Their journeys combine greater path lengths with slower motion, so their orbital periods are longer. Within one elliptical orbit, an object travels faster when closer to the Sun and slower when further away. This is explained using conservation of energy. As the object approaches the Sun, its gravitational potential energy decreases and its kinetic energy increases.

7:19Greater kinetic energy means greater speed for the same object. As the object moves away again, its gravitational potential energy increases while its kinetic energy decreases. Its speed therefore falls, assuming no significant energy losses. Gravity transfers energy between these two stores rather than creating extra energy. Remember that a stronger gravitational pull near the Sun is only part of the explanation; conservation of energy completes it.

Calculating orbital speed

7:44Average orbital speed is the total distance travelled divided by the time taken. For a circular orbit, one complete journey has a distance equal to the circumference. That circumference is 2π times the orbital radius. Average orbital speed therefore equals 2π times the radius, divided by the period. Extended students must recall and use this relationship. The orbital period is the time for one complete orbit. Treat the path as roughly circular and use the average orbital radius.

8:12For a very stretched ellipse, the result is only an estimate of the path length. Our first worked example uses a planet with an orbital radius of one hundred and fifty million kilometres. Its orbital period is three hundred and sixty-five days. We want the average orbital speed in metres per second. We therefore convert both quantities before substituting them into the equation. Multiplying the radius by one thousand gives one hundred and fifty billion metres.

8:37Multiplying three hundred and sixty-five by twenty-four gives eight thousand seven hundred and sixty hours. Multiplying that result by three thousand six hundred gives thirty-one million five hundred and thirty-six thousand seconds. The radius and period are now in suitable units. 2π × 150 billion gives approximately nine hundred and forty-two billion metres. This represents the distance travelled around one approximately circular orbit. Dividing by thirty-one million five hundred and thirty-six thousand gives approximately twenty-nine thousand nine hundred metres per second.

9:10The answer is also about thirty kilometres per second. For a second worked example, a natural satellite follows a circular orbit of radius seven million metres. Its orbital period is six thousand seconds. 2π × 7 million gives approximately forty-four million metres for the circumference. Dividing by six thousand gives an average orbital speed of approximately seven thousand three hundred metres per second. The radius in that calculation is measured from the centre of the body being orbited.

9:37It is not the satellite’s height above the surface. If a question gives height instead, add the planet’s radius to find the distance from its centre. Also keep average speed separate from the changing instantaneous speed in an elliptical orbit.

Light travel time and planetary data

9:49Light travels extremely quickly, but Solar System distances are enormous. To calculate its travel time, use time equals distance divided by speed. In space, the speed of light is approximately three hundred million metres per second. This calculation is Core content, unlike the orbital speed equation. Suppose sunlight travels one hundred and fifty million kilometres to Earth. Converting the distance gives one hundred and fifty billion metres. Dividing that distance by three hundred million metres per second gives five hundred seconds.

10:20Dividing by sixty gives approximately eight point three minutes, so the sunlight arriving now left the Sun several minutes earlier. Analysing planetary data is another Supplement skill. Consider rounded data for Earth and Mars, including distance, orbital duration, density, average surface temperature and surface gravitational field strength. Mars is further from the Sun and has a longer orbital period. Those observations agree with a longer path and a lower average orbital speed.

10:47Earth’s average density is about five thousand five hundred kilograms per cubic metre, while Mars has about three thousand nine hundred. Both values are consistent with rocky planets, though their compositions differ. Density means mass per unit volume, so it does not directly tell us how much matter a planet contains. A larger object can have lower density but greater mass. The displayed average surface temperatures are about fifteen degrees Celsius for Earth.

11:11For Mars, the value is about minus sixty-five degrees. Greater distance generally means less incoming sunlight, but atmosphere also affects temperature. Venus illustrates why temperature need not decrease perfectly in planet order. Always distinguish a general trend from a rule that every data point must follow. Earth’s surface gravitational field strength is approximately nine point eight newtons per kilogram, compared with three point seven on Mars. The same mass therefore experiences a smaller gravitational force on Mars.

11:39Surface field strength depends on both planetary mass and radius. In questions using surface values, treat the field as approximately uniform across the small region being considered.

Exam explanations and recap

11:49Cambridge questions may ask you to describe trends, calculate quantities or explain observations. Describe means state what the data show, while explain requires the physical reason. For a calculation, show the equation, consistent units, substitution and final answer. A result without correct units leaves its physical meaning unclear. Consider an exam-style question asking you to explain why a comet travels faster when closer to the Sun. Its gravitational potential energy decreases as it approaches the Sun.

12:18Its kinetic energy increases because total energy is conserved. Its speed therefore increases, and each sentence gives one creditworthy point. Another question might ask you to explain why the northern hemisphere experiences summer. Earth’s axis is tilted, so the northern hemisphere leans towards the Sun during that part of its orbit. Sunlight is more direct, so incoming energy is concentrated over a smaller area. Daylight lasts longer, increasing the time during which solar energy is received.

12:47The main mistakes come from confusing different motions or quantities. Rotation produces the daily cycle, while the tilted axis and yearly orbit produce seasons. Moon phases come from viewing its illuminated half, not Earth’s shadow. Orbital radius is measured from the centre, and surface gravity cannot be inferred from density alone. Earth’s spin, its yearly orbit and the Moon’s monthly journey explain three different repeating patterns. The Solar System formed through accretion, with gravity gathering material from a rotating cloud into the Sun and planets.

13:17The Sun’s gravitational attraction keeps objects in orbit. Distance affects gravitational field strength, orbital motion and the time taken for light to reach us. For Extended study, connect elliptical motion to conservation of energy and average orbital speed to distance divided by time. Closer to the Sun, an orbiting object gains kinetic energy and moves faster. Across the planets, greater distance generally corresponds to lower orbital speed and a longer period.

13:44These relationships let you interpret unfamiliar data rather than memorise isolated facts.