The hour angles which correspond to the different positions of the Sun in the sky concern the true solar time and not the conventional time measured by a clock. However, true solar time is determined by the Sun’s position in the sky: for example, when the Sun is on the meridian of a certain place, the hour angle is nil because of the solar midday; instead, the clock will indicate a different time.

To convert clock time to true solar time, first, it is necessary to correct the lon­gitude difference between the local meridian and the standard time zone meridian, considering that any grade of longitude difference corresponds to a four-minute correction; second, because the angle speed of the Earth is not constant during the year, it varies positively or negatively as regards the average conventional value of 360/24 grad/h, we have to correct it using the equation of time (ET).

We use the relation [1]:

True solar time = Conventional time + 4′ (Longitude of the local meridian — Longitude of the standard time zone meridian) + ET

If we apply legal time, the conventional hour must not be the legal one, but the one for the standard time zone meridian. Longitudes are considered to be positive if they are eastward from Greenwich. As regards Italy, the datum meridian is at 15° east of Greenwich and it passes by the volcano Etna. Figure 10 shows the ET trends during the year.

Some values of ET are shown in Table 2.