The tower was built between 1578 and 1580 to a design by the Bolognese architect Ottaviano Mascherino (who was credited with building the Apostolic Palace) mainly to promote the study of astronomy for the Gregorian Calendar Reform which was commissioned by Pope Gregory XIII and promulgated in 1582.

The tower was an edifice of great value for astronomical observations made using a sundial as they provided essential confirmation of the need to reform the Julian calendar.

Its instrumentation, apart from many normal devices (such as meteorological and magnetic equipment, with a seismograph and a small transit and pendulum clock,[4]) was noted for the Dolland Telescope.

As an addition, under the patronage of Pope Pius X, four rotary observatory domes were also added at strategic locations on the 1,300 feet (400 m) long fortification walls, more than a thousand years old.

[4] The revival of the observatory on the Gregorian Tower was initiated by the Barnabite Francesco Denza with the approval of Pope Leo XIII.

In 1891, Pope Leo XIII, promulgating the motu proprio Ut mysticam, designated the second tower as the seat of the newly established Vatican Observatory, a decision which required altering the roof to provide a flat terrace for astronomical observations.

[4] As a result of these modifications, the original library was moved to the Pontifical Academy Lincei, and the old meteorological and seismic instruments were shifted to the Valle di Pompei observatory.

Pope Urban VIII had it enclosed and it was subsequently decorated with long sequences of frescoes painted between 1580 and 1582 by Simon Lagi and the two Flemish artists Paul and Matthijs Bril.

[2][8] The Sundial Room, also called the Meridian Hall, was once the residence of Queen Christina of Sweden, then newly converted to Catholicism.

The room was further modified by two additions which gave it its current name: a sundial, and a delicate but sophisticated anemoscope to measure the wind, which was fixed to the ceiling of the Meridian Hall.

The sundial consisted of a straight line in white marble running across the floor in a north–south direction, intended to measure the height of the Sun at noon according to the seasons of the year.

[2][8] The interior walls and ceiling of the hall were richly decorated, in some cases with gaudy frescoes of the hills and Roman countryside, the Pantheons, religious themes, the buildings surrounding the area, and naval shipwrecks with Jesus calming the storm and so forth.