Starwatch: this week’s rare blue moon highlights the strange way we mark our calendar
This week's full moon is referred to as a blue moon, which occurs when there are two full moons in a single calendar month. The term 'blue moon' does not indicate the color of the moon but rather reflects the way we align our calendar with the lunar cycle. While this blue moon will occur on May 31, a stricter astronomical definition suggests that the next true blue moon will not be until May 20, 2027.
- ▪The moon takes 29.5 days to orbit the Earth, which is slightly less than the average month.
- ▪A blue moon is defined as the second full moon in a calendar month.
- ▪This week’s blue moon follows a full moon that occurred on May 1.
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| Original publisher | The Guardian — Science |
| Canonical URL | https://www.theguardian.com/science/2026/may/25/starwatch-this-weeks-rare-blue-moon-highlights-the-strange-way-we-mark-our-calendar |
| Publication time | Mon, 25 May 2026 05:00:16 GMT |
| Retrieval time | 2026-05-25T05:12:36.190Z |
| Last seen | 2026-05-25T05:12:36.190Z |
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Opening excerpt (first ~120 words) tap to expand
The moon takes 29.5 days to circle our planet, slightly less than the average month. Photograph: Danny Lawson/PAView image in fullscreenThe moon takes 29.5 days to circle our planet, slightly less than the average month. Photograph: Danny Lawson/PAStarwatchThe moonStarwatch: this week’s rare blue moon highlights the strange way we mark our calendarBecause we define dates based on the stars, the full moon on 31 May will be the second one of the calendar monthStuart ClarkMon 25 May 2026 01.00 EDTSharePrefer the Guardian on GoogleThis week’s full moon is a blue moon. The term does not describe the colour of the moon, but instead arises from the way we define our calendar in reference to the stars rather than the moon.The moon takes almost a month to circle our planet.
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