Tilted globe around a lamp: seasons and day length

The idea

Earth's axis keeps pointing the same way in space as it orbits, so each hemisphere leans toward the Sun for half the year and away for the other half, changing noon Sun height and day length together.

What you need

• 1 globe on a tilted stand (23.5 degree tilt) or a polystyrene ball on a skewer tilted 23.5 degrees with a protractor
• 1 bare bright LED globe at the centre of a darkened room (incandescent globes for general lighting have not been sold in Australia since 2009)
• 4 masking-tape marks on the floor at 90 degree intervals around the lamp, 1.5 m out, labelled March, June, September, December
• 1 sticky dot on Sydney and 1 on a northern city at a similar latitude north
• 1 torch and 1 sheet of paper to check the terminator (day-night line)

How to do it

1. Set the globe on the December mark with the axis pointing at a fixed wall mark (not at the lamp) and note which hemisphere leans toward the lamp.
2. Spin the globe once and count, with a protractor or by eye against the terminator, roughly what fraction of the spin the Sydney dot is lit; record it.
3. Move the globe to March, June and September keeping the axis pointing at the same wall mark, and repeat the lit-fraction estimate for both dots.
4. Record noon: rotate until the Sydney dot faces the lamp squarely and note how steeply the light strikes the dot at each position.
5. Compare the estimates with the day lengths from the NOAA Solar Calculator for Sydney on 21 December, 21 March, 21 June and 23 September.

What you should see

In December the southern hemisphere leans toward the lamp, Sydney's dot stays lit for well over half a spin and the light strikes it steeply; in June it leans away and is lit for under half a spin at a shallow angle; in March and September the terminator passes through both poles and every dot is lit for half a spin. Computed for Sydney (33.85 degrees S, obliquity 23.44 degrees): the geometric day length (Sun's centre on the horizon) is 14 h 15 min on 21 December, 9 h 45 min on 21 June and 12 h 00 min at the equinoxes; with the standard refraction and solar-disc allowance (zenith 90.833 degrees) the values are 14 h 24 min, 9 h 54 min and 12 h 08 min, which is what the NOAA calculator reports. The learner knows the model worked when the December lit fraction is clearly above one half and the June fraction clearly below, and both hemispheres swap.

What changes

What you change

position of Earth in its orbit (date)

What you measure

fraction of a rotation the site is lit, and the angle of the light at noon

What you keep the same

• axis direction fixed in space
• tilt angle
• lamp height at the globe's centre
• distance from lamp

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• Seasons come from Earth's changing distance from the Sun; the tilt, not the distance, sets them, and the two hemispheres have opposite seasons at the same distance.
• The axis swings to point toward the Sun in summer; the axis direction stays fixed in space while Earth moves around the Sun.
• Day length is 12 hours all year; it varies by more than four hours at Sydney's latitude.

Safety card

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-OTU-01SC4-WS-04SC4-WS-06
• Australian Curriculum v9AC9S7U03AC9S7I01AC9S7I04

Sources

The pages the author read to write this activity.

1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
2. vocabulary.curriculum.edu.au/MRAC/2024/04/LA/SCI/export/MRAC/2024/04/LA/SCI.jsonld
3. spaceplace.nasa.gov/seasons/en
4. nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
5. gml.noaa.gov/grad/solcalc