Why Does a Year Have 365 Days? (Easy Astronomy + Calendar History) Updated 2025
Short answer: Earth needs about 365.2422 days to orbit the Sun. Calendars use 365 and add a leap day some years to stay in sync.
The Orbit in Plain English
One full trip around the Sun takes about 365.2422 days. If we used 365 every year with no correction, seasons would drift earlier and earlier.
Quick tip: The 0.2422 fraction adds up—roughly one extra day every 4 years.
How the Gregorian Rule Fixes Drift
- Divisible by 4 → leap year
- Except divisible by 100 → not a leap year
- Except divisible by 400 → leap year
See the details on How Many Days in a Leap Year?
Takeaway: We average close to 365.2422 days over many years.
Months & Weeks: How It All Fits
- Month lengths: Days in Each Month
- Year → weeks/hours/minutes/seconds: Conversions
- Weeks per year: 52 weeks + extra days
Frequently Asked Questions
Do other planets define different “years”?
Yes. A “year” on any planet is one orbit around the Sun, so length varies with distance and speed.
Is 2025 a leap year?
No. It’s a common year with 365 days.