Astronomy
Age of the Moon
Subtracts the Julian Day Number of a reference new moon and takes the remainder after dividing by the length of the synodic month. The remainder is the Moon's age in days.
The Formula
age = (JDN - epoch) mod 29.530588853
Subtracts the Julian Day Number of a reference new moon and takes the remainder after dividing by the length of the synodic month. The remainder is the Moon's age in days.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| JDN | Julian Day Number | days | A continuous day count for the chosen date. |
| epoch | Reference new moon | days | 2451550.1, the new moon of 6 January 2000. |
| age | Moon age | days | Days since the most recent new moon, from 0 to 29.53. |
What Each Symbol Means
JDNdays
Julian Day Number
A continuous day count for the chosen date.
epochdays
Reference new moon
2451550.1, the new moon of 6 January 2000.
agedays
Moon age
Days since the most recent new moon, from 0 to 29.53.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate the moon phase page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate The Moon Phase, filed under Science. Every number the guide produces comes from substituting into this one line.