The equatorial disk dial has faces on both upper and lower sides. The gnomon points to the north celestial pole and is set perpendicular to the dial plate in the center of the dial. Also, the angle subtended by the ground and the gnomon is set to the same magnitude as the latitude.
Since the Sun is north of the celestial equator half the year and south of the celestial equator the other half, each dial face, which is parallel to the celestial equator, will only receive sunlight for half the year.
The Sun casts a shadow on the upper dial face from 21st March (the spring or vernal equinox) to 22nd September (the fall or autumnal equinox).
On the other hand, the Sun casts a shadow on the lower dial face from 22nd September (the fall or autumnal equinox) to 21st March (the spring or vernal equinox).
Click here for the animation.
On 21st March (spring equinox), the Sun is at a declination of 0º. The gnomon will cast shadows on both the upper and lower faces because the Sun is moving parallel to the dial plate. This is due to the fact that the Sun is not a point in the sky - it subtends an arc of 30' in the sky.
After the spring equinox, the Sun is at a positive declination. The gnomon then casts a shadow only on the upper dial face. This phenomenon will continue till 21st September (autumnal equinox), where the Sun is again moving parallel to the dial plate. The gnomon will then cast shadows on both the upper and lower faces, as in the spring equinox.
After the autumnal equinox, the Sun is at a negative declination. The gnomon then casts a shadow only on the lower dial face. This phenomenon will continue until spring equinox.