The Earth goes round the Sun in about 365¼ days - this is a solar year (but see also Note). Because of the tilt of the Earth on its axis, at one point in its orbit the Sun is over the Tropic of Cancer and at the opposite point in its orbit the Sun is over the Tropic of Capricorn. These times are the Solstices. Twice a year the Sun is over the Equator - these times are the Equinoxes. The Solstices and Equinoxes are, like the astronomical New Moon, brief moments in time not days of the year. It is this tilt of the Earth on its axis which gives us our seasons.
Measuring days and lunar months is very
simple and requires no astronomical instruments. The realisation that the seasons
change in a cyclic pattern is very ancient indeed, but the
measurement of the length of the solar year requires very careful
observations of the positions of Sunrise and Sunset every day over
very long periods of time (at least ten years). These measurements
were first made by Egyptian priests (Reference 6). This
is because the entire life of Ancient Egypt revolved round the
River Nile and the annual inundation. They needed to be able
to predict this with great accuracy because if the irrigation ditches
were dug too soon they would crumble away in the hot
sun, but if they were dug too late they would be
no use. (Ref 1).
They had reached
a figure of 365 days for the length of the solar
year by 4236 BCE, and a figure of 365¼ days by before
3000 BCE. By about 2500 BCE they had measured the length
of the solar year to an accuracy of, in modern units,
better than ten minutes, that is, accurately enough to have given
them the Gregorian Calendar! (This is discussed next, in The Julian
Calendar.)
The Egyptian Calendar is discussed, mainly for children so more
simply but at greater length, elsewhere in this web site. To
link to it click here 
You can read more about early calendars, and also clocks and time-keeping, by visiting the A Walk Through Time website.
For the most accurate measurements of the length of the solar year we must specify precisely what time interval we are measuring. This is a tropical year. This starts and ends at the instant at which the Sun passes over the equator travelling from South to North.