On account of the unequal motion of the Earth in its orbit and the inclination of the orbit in regard to the equator, the length of days varies with the seasons of the year.
Until the end of the XVIIth century, astronomers used the time of the apparent solar day (time elapsed between two consecutive transits of the Sun across the local meridian) for recording their observations. Once agreement had been reached on the corrections to be made, they then adopted, after the example of Flamsteed, the mean solar time (time referring to the apparent path across the sky of a "fictitious sun" moving with mean velocity along the ecliptic). The analemma - the large figure 8-shape that usually wrapped round the meridian lines - indicates the varying of the correction with the months of the year (identified by zodiacal signs), making possible immediate visualization of the so-called "Equation of time", i.e. the difference between apparent solar time and mean solar time.