索尔吉奈特克斯的巨石墓，巴斯克地区，阿拉瓦省，西班牙 Dolmen of Sorginetxe, Basque Country, Álava, Spain (© David Herraez Calzada/plainpicture)

索尔吉奈特克斯的巨石墓，巴斯克地区，阿拉瓦省，西班牙 Dolmen of Sorginetxe, Basque Country, Álava, Spain (© David Herraez Calzada/plainpicture)

天空中的圆周率 Pi in the sky

庆祝圆周率日

计算圆的周长与直径的比值就像 π 一样简单。今天是圆周率日，一个专门庆祝这一数学常数的节日。这个日期之所以被选中，是因为3、1、4正是圆周率π的前三位有效数字。作为一个无理数，圆周率无法被写成分数，其小数部分会无限延伸且不重复。人类对 π 的探索可以追溯到公元前 1900 年左右的古巴比伦和古埃及。

然而，圆周率日的正式起源相对较晚。20 世纪 80 年代，物理学家拉里·肖在旧金山一家科学博物馆组织了首次圆周率日的庆祝活动。当天，工作人员围着圆圈行走，品尝与 π（Pi）谐音的馅饼（Pie），以此象征这一数学常数。今日图片带我们来到西班牙北部的阿拉瓦，星空在索尔吉奈特克斯的巨石墓上空旋转划出圆形轨迹。这座史前巨石遗迹可追溯至公元前 3000 年。自古以来，圆周率（π）在天文学研究中扮演着重要角色，印度和波斯的天文学家曾计算出最早且最精确的圆周率近似值，为人类探索宇宙奠定了基础。

Celebrating Pi Day

Calculating the ratio of a circle's circumference to its diameter is as easy as pi. Today is Pi Day, an event that celebrates this mathematical constant. This date was chosen because 3, 1, and 4 are the first three significant figures of pi (π) when it is written out. Pi is an irrational number, which means that it can't be written as a fraction. When written out, its decimal representation goes on infinitely without repeating. The oldest attempts to calculate pi date back to ancient Babylon and Egypt, around 1900 BCE.

Pi Day, however, has more recent origins. In the 1980s, physicist Larry Shaw organized the first celebration, where staff at a San Francisco science museum walked around in circles and then ate the number's delicious homophones: pies. Today's image takes us to Álava, northern Spain. The circular motion of the stars appears above the Dolmen of Sorginetxe, an ancient megalithic monument dating back to 3000 BCE. Pi has long played a key role in the study of the stars, and some of the most accurate early approximations of the number were calculated by Indian and Persian astronomers.