Skydomes and seasons
Classroom Activity for 11-14 14-16
What the Activity is for
This activity uses a globe with a transparent dome to explore why day length and the path of the Sun across the sky varies with the seasons by comparing the March equinox and June solstice.
What to Prepare
This activity works best in a darkened room. Equipment required for demonstration (or per group of students):
- Approx. 40 cm diameter globe
- Small transparent dome (eg half of a 4 cm clear plastic bauble)
- Blu Tack or sticky tape
What Happens During this Activity
- Attach the dome to the globe, covering the UK and Ireland.
- Hold the globe about 1 m from the lamp (the Sun) so that the globe’s equator and the centre of the lamp are at similar heights.
- Estimate day length on the March equinox: (a) tilt the globe so that the North Pole points away from the direction of travel (see A on diagram); (b) spin the globe anticlockwise about its axis until the reflection of the lamp just appears on the base of the eastern edge of the dome. This is the sunrise position; (c) continue to spin the globe so that the Sun travels up the dome and sets on its western edge. Estimate the angle that the globe rotated through from sunrise to sunset.
- Estimate day length on the June solstice: (a) move the Earth to its position three months later by walking a quarter of a circle anticlockwise around the lamp. The axis should now point towards the lamp; (b) put the globe in the sunrise position (see B on diagram). Spin it anticlockwise and estimate the angle that it rotated through from sunrise to sunset.
- This sun path diagram shows the apparent path of the Sun across the sky. Link this to the activity by asking students to imagine what they’d see if they were inside the dome on the globe on the March equinox and the June solstice.
- Students should see that on the March equinox, a day corresponds to half a rotation of the globe, ie day length is 12 hours. On the June solstice it requires more than half a rotation, ie day length is greater than 12 hours.
- They should also see that on the March equinox the Sun rises in the east and sets in the west, while on a June solstice, it rises in the northeast, travels higher across the sky and sets in the northwest.
- Ask your students to think about how the seasons coincide with day length. It is warmer in June because the days are longer (and also because the angle of the sunlight is steeper). But summer only begins after the June solstice because the longest day is not the warmest; it takes a while for the atmosphere to warm up. In fact, July and August are the warmest months in the UK and Ireland.
- Equinox: the date on which day and night last equal times across the world. Equinoxes occur in March and September, when the Earth’s tilt is aligned to its direction of travel. The March equinox marks the first day of spring in the northern hemisphere and the first day of autumn in the southern hemisphere.
- Solstice: the longest day of the year in one hemisphere, and the shortest in the other. Solstices occur in June and December when the Earth has its maximum tilt towards/away from the Sun. The June solstice is the first day of summer in the northern hemisphere and the first day of winter in the southern hemisphere.
For a demonstration of how the surface temperature of the Earth varies according to the angle of sunlight:
Students can explore day, night and seasons on other planets in activity five of Exoplanet Physics at iop.org/exoplanets.
Originally published as a Teaching Tip in Classroom Physics March 2019.