Calendars and the beginnings of astronomy (KOR)

Because of those remainders and that elliptical path, someone had to scrutinize the motions of the sun, moon and stars, grappling with calculations. Astronomy began that way.

Moon Hong-kyu

The author is a principal researcher at the Korea Astronomy and Space Science Institute.

Only a few days have passed since I opened a new calendar. The days yet to come are still wound tight, like a spring. Numbers burst open each date as if a cuckoo were leaping from a clock to announce the day. Everyone knows that a day is the time it takes the Earth to rotate once with respect to the sun, and a year is the time it takes to complete one orbit. The problem begins after that.

An employee makes calendars for 2026, the Year of the Fire Horse, at a calendar printing shop in Jangan District, Suwon, Gyeonggi Province, on Nov. 3, 2025. [NEWS1]

Multiply 365 days by 24 hours and it should come out neatly to 8,760 hours. Yet, like coffee grounds stuck to the bottom of a cup, there remains an extra 5 hours, 48 minutes and 45.2 seconds in a year around the sun. That is why Feb. 29 is added and removed every four years.

Perhaps my eyesight has worsened again. I squint, glasses perched on the tip of my nose, trying to make out the tiny lunar calendar numbers. A lunar month, from full moon to full moon or from new moon to new moon, averages 29 days, 12 hours, 44 minutes and 2.9 seconds. Once again, there is leftover change in a year, about 13 hours.

The moon travels along an elliptical orbit, sometimes pulling away to a distance that is three or four times Earth’s diameter farther than at its closest approach. As a result, each lunar month can vary by about seven hours from year to year. A supermoon is simply a full moon seen when the moon is closer to Earth.

What if a day, a month and a year fit together as precisely as interlocking gears, fixed at 24 hours, 30 days and 360 days, with the moon gliding elegantly along a perfect circle? Because of those remainders and that elliptical path, someone had to scrutinize the motions of the sun, moon and stars, grappling with calculations. Astronomy began that way.

Late last year, an overseas researcher visited the institute to present studies of exoplanets observed with the James Webb Space Telescope. The research captured signals of dimming light when so-called super-Earths and mini-Neptunes, with masses ranging from 1.2 to 9.6 times that of Earth, passed in front of their host stars.

Those worlds are heated to extremes, from temperatures comparable to grilling fish to conditions near molten lava. The likelihood of life there appears slim. Their years last anywhere from a day and a half to just 17 days as they skim close to their stars. Many are likely tidally locked, with their rotation period matching their orbital period so that a day and a year are the same.

In places where clocks and calendars coincide, life, too, might be correspondingly simple.

달력문홍규 한국천문연구원 책임연구원

새 달력을 펼친 지 며칠 되지 않았다. 달력에는 아직 오지 않은 날들이 태엽처럼 감겨있다. 쿠쿠 시계 속 뻐꾸기처럼 숫자들이 문을 박차고 하루를 연다.

하루는 지구가 태양을 기준으로 자전하는 시간, 한 해는 공전하는 시간이라는 건 다 안다. 문제는 그다음부터다. 365일에 24시간을 곱하면 8760시간으로 딱 떨어질 법한데, 찻잔 바닥에 눌어붙은 커피 원두처럼 5시간 48분 45.2초가 남는다. 2월 29일을 넣다 뺐다 하는 이유다.

그새 눈이 더 흐려진 걸까? 깨알 같은 음력 숫자를 보려고 코끝에 안경을 걸친 채 미간을 찌푸린다.

음력 한 달은 보름에서 보름, 그믐에서 그믐까지로 평균 29일 12시간 44분 2.9초. 잔돈처럼 또 13시간이 남는다. 달은 타원 궤도를 공전해 지구 지름 서너 배까지 거리가 벌어진다. 그 결과, 해마다 음력 한 달에 7시간 안팎의 편차가 나타난다. 수퍼문은 지구에 가까울 때 우리가 보는 보름달이다.

하루·한 달·1년이 24시간·30일·360일로 톱니바퀴처럼 정교하게 들어맞는 동시에 우아하게 달이 원 궤도를 돈다면? 그 ‘나머지’와 타원 궤도 덕분에 누군가는 해·달·별의 운동을 샅샅이 살펴, 머리를 싸매고 계산해야만 했다. 천문학은 그렇게 시작됐다.

지난해 말, 해외 연구자가 천문연구원에 와 제임스웹 우주망원경(JWST)으로 관측한 외계행성 연구를 소개했다. 지구 질량 1.2배에서 9.6배에 이르는 ‘수퍼 지구’와 ‘미니 해왕성’이 그들의 모성 앞을 지날 때 광도가 떨어지는 신호를 잡은 것. 그곳은 생선 굽는 온도에서 용암 주변 기온에 이를 만큼 달궈져, 생명이 살 확률은 희박하다.

1년은 하루 반나절에서 17일까지로 그 모성을 스치듯 공전해, 하루와 한 해가 같은 ‘동주기 자전’을 할 확률이 높다. 시계와 달력이 같은 그곳에서는, 삶도 그만큼 단출하리라.

This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.