SME
Grade 12 Science · Problem type
Day length from fossil coral
Use this for “A fossil coral shows 1,200 daily growth lines across 3 yearly bands. How long was a day then?”
Diagram
1,200
daily
lines
across
3
yearly
bands:
1,200
3
=
400
days
in
a
year.
A
year
is
still
8,766
hours,
so
each
day
lasted
8,766
400
=
21.915
hours.
This is an example. Type your own values to start a new problem.
D (hours)
=
8,766
hours
÷
(
n
÷
b
)
n ÷ b is the days in one year, N. A year is 8,766 hours (365.25 × 24), so one day was 8,766 ÷ N hours.
N
Days in a year
calculated · days
Formulas
N
=
n
b
400
=
1,200
3
D
=
8,766
N
21.9
=
8,766
400
Assumptions
•
A coral adds one thin growth line a day and one band a year.
•
The year’s length in hours has not changed; the Moon’s tides slow Earth’s spin.
•
So long ago a year had more days, and each day was shorter.
Step-by-step
WE KNOW
n = 1,200, b = 3
FIND
days in a year (N), length of a day (D)
Step 1 · Find days in a year (N)
Use N = n ÷ b
Share the daily lines among the yearly bands: the days in one year.
N
=
1,200
3
N
=
400
days
Step 2 · Find length of a day (D)
Use D = 8,766 ÷ N
A year is 8,766 hours; share them among its days.
D
=
8,766
400
D
=
21.9
hours
Check
Put the numbers back in. Both sides should match.
400
=
1,200
3
✓
21.9
≈
8,766
400
✓
← Previous lesson
Earth's history: the early Earth, its atmosphere and the history of life
Next lesson →
How oxygen filled the air
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
Related lessons
Earth's history: the early Earth, its atmosphere and the history of life
Main lesson
How oxygen filled the air
Use this for “How did the first photosynthesis change Earth’s air, and what did it leave in the rocks?”
Early Earth’s air and today’s
Use this for “How was Earth’s early atmosphere different from today’s, and what changed it?”
The history of life
Use this for “Put the major steps in the history of life in order, from the first cells to humans.”