大年也称为柏拉图年或岁差年，是春分点完整的绕行黄道一周所经历的时间，长度大约是25,800年，目前较精确的数值是25,765年，因为进动的数值会改变，所以无法计算出更准确的时间长度。虽然占星学很重视大年，但现代的天文学家却不太在意。岁差的受到重视是源自于学者对神话起缘的研究，例如对罗伯特坟 (Robert Graves)综合性的研究，就包含了希腊神话。经由考古学从早期的历史纪录和史前史符号中的发现，可以从更早的神话片段中寻找到它们的起源。

在文学作品中发现的岁差周期通常都是26,000年，都围绕着更精确的25,800年的周期。因为岁差的变化数值是随着时间改变的，而实际上不可能测量出过去改变的确切速率。现在采用的数值是每年50.3″角秒，因此每25,765年完整的环绕一周，但实际的数值在50.25″至50.34″之间变动者，因此环绕一周的时间会与以50.3″角秒推算的有所出入，可能会在25,791年至25,744之间。

The term Great Year has a variety of related meanings. It is defined by NASA as "The period of one complete cycle of the equinoxes around the ecliptic, about 25,800 years […] also known as [a] Platonic Year." One complete cycle of the equinoxes here means one complete cycle of axial precession; this precession was known to Plato, who defined the "perfect year" as the return of the celestial bodies (planets) and the diurnal rotation of the fixed stars (circle of the Same) to their original positions. Cicero followed Plato in defining the Great Year as a combination of solar, lunar and planetary cycles (61 XX) Nicholas Campion writes of "periods of History, analogous to the solar year, known as 'Great Years' "