Here are the markers for the measurements last taken in 1817:
(left-above) Inscription on the northern side of the obelisk stating that the sundial measurements were taken in 1817; (left-below and centre) marks of the sundial at noon; (right-above) mark of the southern colonnade focus; (right-below) one of the eight marks showing the cardinal points and the direction of winds
The obelisk was erected in Heliopolis most likely by Pharaoh Amenhemat II (it does not have any inscription in hieroglyphics); it was relocated to Alexandria by the Romans at the time of Emperor Augustus. It was brought to Rome by Emperor Caligula who placed it in the private circus he had built at the Vatican (the circus was later on named after Emperor Nero). The obelisk was the only one which did not fall down and this fact in the Middle Ages was attributed to the reputation of Nero as a sorcerer. It was topped by a (lost) bronze globe which was thought to contain the ashes of Julius Caesar (to see all the obelisks of Rome click here).
In 1586 Pope Sixtus V ordered Domenico Fontana to relocate the obelisk from the southern side of the old basilica, near Sacrestia di S. Pietro to the square in front of it. At the time the dome was not yet completed and the obelisk was not perfectly aligned with it. The top of the obelisk was decorated with a bronze cross above a star and three mountains (other heraldic symbols of the pope); in three inscriptions Pope Sixtus V celebrated the relocation of the obelisk and its change from a symbol of the pagan world (ab impura superstitione) to the holder of the Holy Cross; the text of the front inscription (which means Behold the Cross of the Lord. Flee ye adversaries! The Lion of the Tribe of Juda has won) is known as St. Anthony's Brief and was used in exorcisms.
In 1817, in imitation of Augustus' sundial in Campo Marzio, the shadow of the obelisk was measured and since then it serves as a gnomon and its shade at noon indicates the day of the year. The starting days of the zodiacal months are indicated by circular slabs of marble; similar slabs were also used to indicate the cardinal points and the focuses of the two colonnades. It is likely that the leaning of the obelisk was corrected at this time.