Earth’s Rotation & Orbit
How long does it take the Earth to rotate once on its axis? (in minutes)
1440 minutes
How long does the Moon take to orbit the Earth?
About one month
Name the two groups of planets and one key physical difference.
Inner (Mercury–Mars) – rocky, smaller, dense;
Outer (Jupiter–Neptune) – gaseous, bigger, low density
What determines the strength of a planet’s gravitational field? Provide 2 reasons.
Mass of the planet and distance from its centre
Average orbital speed formula.
v = 2πr / T
Explain why the Sun appears to move across the sky during the day.
Because the Earth rotates on its axis
Explain why we see different phases of the Moon.
Because we see varying fractions of the sunlit side as the Moon orbits Earth
Why do planets orbit the Sun?
Gravitational attraction provides centripetal force
A satellite is in orbit around Earth. Why do astronauts inside it not feel their usual weight?
They are in constant free fall along with the satellite; gravity acts, but there is no support force from a surface.
Earth: r = 1.5×10¹¹ m, T = 365 days. Calculate v.
v ≈ 29,800 m/s
How does the tilt of the Earth’s axis cause seasons?
Different hemispheres receive varying amounts of sunlight during the year
Write the formula for average orbital speed and define each symbol.
v = 2πr / T
v = orbital speed,
r = orbital radius,
T = orbital period
Explain why a planet moves faster when closer to the Sun in its elliptical orbit.
Gravitational potential energy converts to kinetic energy (energy conservation)
Why do satellites in low Earth orbit move faster than geostationary satellites?
Stronger gravitational pull at lower altitude → higher centripetal force → faster speed
How long does light take to travel from Sun to Neptune? (r = 4.5×10¹² m, c = 3×10⁸ m/s)
t ≈ 15,000 s ≈ 4.2 hours
Explain why the length of daytime changes throughout the year.
Tilt of the axis changes Sun’s path in the sky; summer → longer days, winter → shorter days
Why does the Moon always show the same face to Earth?
It is tidally locked; its rotation period = orbital period
How does distance from the Sun affect a planet’s orbital period and speed?
Farther → weaker gravity → slower speed → longer period
Why does a planet in an elliptical orbit not move at constant speed?
Because the distance from the Sun changes; gravitational potential energy and kinetic energy are exchanged, so speed varies.
Planet: r = 2×10¹¹ m, T = 2 years. Find average orbital speed.
v ≈ 19,900 m/s
If the Earth’s axis tilt were zero, what effect would this have on seasons?
No seasons; temperature and day length would be nearly constant throughout the year
Explain why we see moon phases every month but solar eclipses only occasionally.
Moon’s orbit is tilted ~5° to Earth–Sun plane, so perfect alignment occurs only at nodes
Explain why the Sun is not at the centre of a planet’s orbit.
In an elliptical orbit, the Sun is at one focus, not at the centre, due to the way gravity and initial motion shape the orbit.
How does the Sun’s gravitational field affect the orbital speed of planets?
Weaker field farther from Sun → slower orbital speed; stronger field closer → faster speed
A comet moves faster at one part of its orbit than at another. Which part is it, and why?
It moves fastest at perihelion (closest to the Sun) because gravitational potential energy is converted into kinetic energy.