The Earth, Moon and SunIB MYP Physics: Revision notes
Section 1
Rotation: day and night
The Earth rotates on its axis once every 24 hours, turning from west to east. The side facing the Sun has day and the side facing away has night. The Sun appears to rise in the east and set in the west because of this rotation, not because the Sun moves around us.
Section 2
Orbit and the year
The Earth orbits the Sun once in about 365¼ days, which is one year. Its orbit is nearly circular. The extra quarter day is why we add a leap day every four years.
The Earth's orbital period is 1 year and its rotation period is 1 day. Do not mix them up.
Section 3
The seasons
The Earth's axis is tilted by 23.5°. As the Earth orbits the Sun, the tilt always points the same way in space. So for part of the year the northern hemisphere is tilted towards the Sun and part of the year away.
- Tilted towards the Sun: the Sun is higher in the sky, sunlight is more concentrated, and days are longer. This is summer.
- Tilted away: the Sun is lower, sunlight is spread out, and days are shorter. This is winter.
The seasons are opposite in the northern and southern hemispheres.
Seasons are not caused by the Earth being closer to or further from the Sun. Its distance changes very little. The tilt is the cause.
Section 4
Phases of the Moon
The Moon does not make light. It reflects sunlight. The Sun always lights half of the Moon. The Moon orbits the Earth about once a month, and as it moves we see different amounts of the lit half. This gives the phases, which repeat about every 29.5 days:
new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, waning crescent.
At new Moon the Moon is between the Earth and the Sun, so the lit half faces away from us. At full Moon the Earth is between the Sun and the Moon and we see the whole lit half.
The phases are not caused by the Earth's shadow. The Earth's shadow only falls on the Moon in a lunar eclipse.
Section 5
Solar and lunar eclipses
A solar eclipse happens at new Moon when the Moon passes between the Sun and the Earth. The Moon's shadow falls on a small part of the Earth, where the Sun is blocked.
A lunar eclipse happens at full Moon when the Earth is between the Sun and the Moon. The Earth's shadow falls on the Moon.
Eclipses are not monthly because the Moon's orbit is tilted slightly compared with the Earth's orbit around the Sun, so the three bodies are rarely lined up exactly.
Section 6
Tides
Tides are the regular rise and fall of the sea. They are mainly caused by the gravitational pull of the Moon (and the Sun) on the oceans. The Moon's pull makes a bulge of water (high tide) on the side nearest the Moon, and a second bulge on the opposite side. As the Earth rotates, a coast passes through two high tides and two low tides each day.
When the Sun, Earth and Moon are in line (new and full Moon) their pulls add together, giving the biggest tidal range. At half Moon the pulls partly cancel, giving the smallest range.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The Earth, Moon and Sun
- A student in Reykjavik, Iceland, notices that in June the Sun stays above the horizon for about 21 hours each day, but in December it is up for only about 4 hours. Reykjavik is in the northern hemisphere.Explain why Reykjavik has much more daylight in June than in December.2 marks
- Astronomers at an observatory in Chile record the appearance of the Moon every night. Over about 29.5 days its visible shape changes from a thin crescent to a full circle and back again. They also note that half of the Moon is always lit by the Sun.Explain why half of the Moon is always lit but the shape we see changes over the month.2 marks
- A total solar eclipse is predicted to cross parts of Indonesia. About two weeks later, a total lunar eclipse will be visible across Africa and Europe. Eclipses do not happen every month, even though the Moon orbits the Earth about once a month.Describe how a solar eclipse happens.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).