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Often, there is a note or caption which explains it.
Otherwise, the sign can be determined by knowing that, during the first three months of each year, the clock is ahead of the sundial.
There is no universally accepted definition of the sign of the equation of time.
Some publications show it as positive when a sundial is ahead of a clock, as shown in the upper graph above; others when the clock is ahead of the sundial, as shown in the lower graph.
The mnemonic "NYSS" (pronounced "nice"), for "new year, sundial slow", can be useful.
Some published tables avoid the ambiguity by not using signs, but by showing phrases such as "sundial fast" or "sundial slow" instead.
Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would resolve to zero.
When good mechanical clocks were introduced, they agreed with sundials only near four dates each year, so the equation of time was used to "correct" their readings to obtain sundial time.
Some clocks, called equation clocks, included an internal mechanism to perform this "correction".
Later, as clocks became the dominant good timepieces, uncorrected clock time, i.e., "mean time" became the accepted standard.
The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.
During a year the equation of time varies as shown on the graph; its change from one year to the next is slight.