noun /ˈsɒl.stɪs/
One of the two points in the ecliptic at which the sun is furthest from the celestial equator. This corresponds to one of two days in the year when the day is either longest or shortest.
For all men begin, as we said, by wondering that things are as they are, as they do about self‐moving marionettes, or about the solstices or the incommensurability of the diagonal of a square with the side;
See Also: equinox
Wikipedia foundation.