Lookback Time Calculator Formula
Understand the math behind the lookback time calculator. Each variable explained with a worked example.
Formulas Used
Lookback Time
lookback_myr = distance_mpc * 3.26156Lookback Time (Gyr)
lookback_gyr = distance_mpc * 3.26156 / 1000Variables
| Variable | Description | Default |
|---|---|---|
distance_mpc | Distance(Mpc) | 100 |
How It Works
Lookback Time
Light travels at a finite speed, so observing a distant object means seeing it as it was in the past.
Simple Estimate
t ≈ d / c
where d is in light-years and t in years. Since 1 Mpc ≈ 3.262 Mly, the lookback time in millions of years equals the distance in Mpc × 3.262.
For very large distances, cosmological expansion makes this an underestimate of the true lookback time.
Worked Example
A galaxy is 100 Mpc away.
distance_mpc = 100
- 01t ≈ 100 Mpc × 3.262 Mly/Mpc = 326.2 Mly
- 02We see this galaxy as it was 326 million years ago.
- 03In Gyr: 0.326 Gyr
Ready to run the numbers?
Open Lookback Time Calculator