1.
moon age; lunar age; number of days since the new moon
The number of days elapsed since the last new moon, used to describe the current phase of the moon.
今日の月齢は十五だ。
Today's moon age is fifteen.
月齢によって潮の満ち引きが変わる。
The tides change depending on the lunar age.
釣りに行くときは、月齢を確認してから計画を立てる。
When going fishing, I check the moon age before making plans.
2.
age in months (of a baby or infant)
The age of a baby measured in months rather than years. Commonly used in childcare and pediatric contexts.
月齢六か月の赤ちゃん。
A six-month-old baby.
月齢に合った離乳食を選ぶ。
To choose baby food appropriate for the baby's age in months.
この予防接種は月齢三か月から受けられます。
This vaccination can be given from three months of age.
A word with two distinct usage contexts: astronomy/nature and childcare. The astronomical sense is important for fishing, gardening, and traditional calendar activities.
SENSE 1 — ASTRONOMY:
月齢 counts the days from the new moon (新月, 月齢 0). A full moon is approximately 月齢 15. Fishermen, gardeners, and tide watchers use this information regularly.
SENSE 2 — CHILDCARE:
In Japanese childcare, babies' ages are often given in months (月齢) rather than years until they turn one or two. Developmental milestones and vaccination schedules are typically described by 月齢.
SIMILAR WORDS:
- 月例: monthly (event) — a homophone with different kanji; means 'regular monthly' (e.g., 月例会議 monthly meeting)
- 年齢: age (in years) — the standard word for a person's age
- 週齢: age in weeks — used in animal husbandry and early infant care