Exercises: Seasons, lunar phases and eclipses

The Earth's axis is tilted at about $23.5^\circ$ to the vertical as it orbits the Sun.

1. a) State how long the Earth takes to complete one orbit of the Sun.

1. b) State which season Ireland has in June and which season it has in December.

1. c) Name the feature of the Earth that causes the seasons.

Name the four main phases of the Moon in the order they appear, starting with the New Moon, and state approximately how many days pass between one New Moon and the next.

1. a) Name the type of eclipse that happens when the Moon passes between the Sun and the Earth.

1. b) Name the type of eclipse that happens when the Earth passes between the Sun and the Moon.

1. c) State which of these two eclipses happens only at New Moon and which only at Full Moon.

In Dublin, the Sun is high in the sky at noon in June and low in the sky at noon in December. Day length is about 17 hours in June and about 7.5 hours in December.

1. a) Calculate the difference in day length between the two months.

1. b) Explain, using the tilt of the Earth's axis, why the days are longer in June.

1. c) Explain why the Sun's rays heat the ground more effectively when the Sun is high in the sky.

A student in Galway looks at the Moon on a clear night. She sees that the right-hand half of the Moon's face is lit and the left-hand half is dark.

1. a) Name this phase of the Moon.

1. b) Describe the positions of the Sun, Earth and Moon at this phase.

1. c) Explain why we always see the same face of the Moon from Earth, using the fact that the Moon rotates once per orbit.

1. d) Explain why the Moon is visible at all, given that it does not produce its own light.

A solar eclipse is visible from part of the west of Ireland. A Moon of diameter 3,474 km is about 384,000 km from the Earth. The Sun has a diameter of about 1,391,000 km and is about 150,000,000 km from the Earth.

1. a) Calculate the ratio of the Sun's diameter to the Moon's diameter, to the nearest whole number.

1. b) Calculate the ratio of the Sun's distance to the Moon's distance, to the nearest whole number.

1. c) Use your answers to explain why the Moon can just cover the Sun's disc in the sky.

A student claims: "We have seasons because the Earth is closer to the Sun in summer and further away in winter." The Earth is about 147 million km from the Sun in early January and about 152 million km from the Sun in early July.

1. a) Use these data to show that the student's claim is incorrect for Ireland.

1. b) Explain the real cause of the seasons, referring to the tilt of the axis, the angle of the Sun's rays and day length.

1. c) Predict the season in Australia when it is summer in Ireland, and justify your answer.

The Moon's orbit is tilted by about $5^\circ$ relative to the plane of the Earth's orbit around the Sun. A solar eclipse can occur only at New Moon, and a lunar eclipse only at Full Moon, yet there is a New Moon and a Full Moon every month.

1. a) Explain why eclipses do not happen every month.

1. b) Explain why a lunar eclipse can be seen from every place on Earth where the Moon is above the horizon, whereas a total solar eclipse is visible only along a narrow path.

1. c) During a total lunar eclipse the Moon often appears dull red rather than disappearing. Suggest an explanation.

Between two consecutive Full Moons about $29.5$ days pass, but the Moon takes only about $27.3$ days to orbit the Earth once relative to the stars.

1. a) Calculate the difference between these two periods.

1. b) Explain, with reference to the Earth's movement around the Sun, why the time between two Full Moons is longer than the orbital period of the Moon.

1. c) A Full Moon occurs on 1 March. Estimate the date of the next Full Moon and of the next New Moon after 1 March.