Stars return about 4 minutes earlier each night: the sidereal day and the Emu in the Sky

The idea

Because Earth moves along its orbit, a star comes back to the same place in the sky 3 minutes 56 seconds earlier each night, so the evening sky shifts through the year, a cycle that Aboriginal and Torres Strait Islander peoples have long used as a calendar.

Safety card

What you need

• a fixed sighting line, such as the edge of a building or a vertical pole viewed from a marked spot, used from the same spot every night
• a phone or watch showing network time to the second
• a star chart or free planetarium software set to the date and the school's latitude, to pick a bright star (for example Acrux in the Southern Cross, or Antares) and to find the dark-cloud Emu in the Milky Way
• an observing log with columns for date, star and clock time at the sighting line
• a torch covered with red film, warm clothing, and a parent or carer present

How to do it

1. Choose a bright star that reaches the sighting line in the early evening.
2. On the first night record, to the second, the time it reaches the line.
3. Repeat from the same spot on at least five nights spread over two to four weeks; if daylight saving starts or ends in between (in NSW it starts on the first Sunday in October), subtract or add an hour.
4. Plot clock time against date and find the gradient in minutes per day.
5. Compare the gradient with 24 h minus the sidereal rotation period of 23.9345 h from the NASA Earth Fact Sheet.
6. In two different months (for example April or May, and again in August or September, two of the times Fuller and colleagues describe) look for the Emu in the Sky in the early evening, compare its position and shape, and read what Fuller and colleagues (2014) recorded from Kamilaroi and Euahlayi knowledge; where the school can, invite a local Aboriginal knowledge holder through its Aboriginal Education Consultative Group contact.

What you should see

Each night the star reaches the line 3.93 minutes (3 minutes 56 seconds) earlier by the clock, 117.9 minutes (just under 2 hours) earlier after 30 days, and the sky turns 0.99 degrees further west at the same clock time each day (360 degrees in 365.256 days). A learner's gradient within 0.3 minutes per day of 3.93 shows the method worked. Fuller and colleagues (2014) record that for the Kamilaroi and Euahlayi peoples the Emu's changing shape and position in the evening sky through the year are tied to emu breeding and the time when eggs can be collected.

What changes

What you change

date of observation

What you measure

clock time the star reaches the sighting line

What you keep the same

• same observing spot and sighting line
• same star
• clock set to network time

Common misconceptions

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

• A star rises at the same clock time every night; it rises about 4 minutes earlier each night.
• The night sky looks the same all year; the evening sky changes with the seasons because Earth moves along its orbit.
• Aboriginal astronomy is only stories; it rests on careful, repeated observation used to time seasonal events.

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-01SC4-WS-05SC4-WS-06
• Australian Curriculum v9AC9S7U03AC9S7H02AC9S7I03AC9S7I05

Sources

The pages the author read to write this activity.

1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/overview
3. vocabulary.curriculum.edu.au/MRAC/2024/04/LA/SCI/export/MRAC/2024/04/LA/SCI.jsonld
4. nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
5. arxiv.org/abs/1403.0304