Geminos wrote an introduction to astronomy where he mentioned both Hipparchos and Hero as forerunners, although he is assumed older than Hero. This problem would be solved by redating Hipparchos (and Geminos accordingly).

Hipparchos was a Greek astronomer and mathematician said to have lived about BC 190 - 120. Ptolemy attributes to him astronomical observations in the period from 147 BC to 127 BC. Some of these are stated as made in Rhodes. Some earlier observations since 162 BC might also have been made by Hipparchos.

Although he wrote at least fourteen books, relatively little of Hipparchos' direct work survived into modern times (ref 1). However, there are a few observations probably made by Hipparchos himself, which are mentioned by later authors and are detailed enough to give us a possible dating.

The solar eclipse. The first is a solar eclipse used by Hipparchos to determine the size and distance of sun and moon.

Cleomedes writes: "Once when the Sun was wholly eclipsed in the Hellespont, it was observed in Alexandria to be eclipsed except for the fifth part of its diameter."
We are looking for a solar eclipse which is total in the Hellespont region and of magnitude 0.8 in Alexandria, occuring during the active time of Hipparchos as an astronomer. There are five matching eclipses in the time span -199 to 200 (ref 2)!

The eclipse of -128 Nov. 20 is regarded by many as the most likely match for this observation. It is total in the Hellespont and has a magnitude of 0.79 in Alexandria. But, it comes very late in Hipparchos' active life.

We might now redate Hipparchos by 232 years towards our time according to our previous hypothesis. If this is right he most likely has been active as an astonomer between AD 70 to 105. One of the eclipses matching the observation is within this time span, on AD 71 Mar. 20! If this is Hipparchos' eclipse, he would have observed it when he was 29 years old.

The lunar eclipse. The second observation is a lunar eclipse described by Pliny as follows:
"He also discovered for what exact reason, although the shadow causing the eclipse must from sunrise onward be below the earth, it happened once in the past that the moon was eclipsed in the west while both luminaries were visible above the earth." (ref 3)
We are looking for a total lunar eclipse visible in Greece or Asia Minor, where the moon is setting eclipsed exactly at sunrise.
This is a typical midwinter observation. The nearer to winter solstice the better and longer the observability, probably also enhanced by refraction near the horizon line. The frequency of such an occasion is about 1 to 4 per century.

Toomer (ref 6) argued that this must refer to the large total lunar eclipse of -138 Nov. 26, when over a clean sea horizon as seen from Rhodes, the Moon was eclipsed in the northwest just after the Sun rose in the southeast.

If Hipparchos has lived 232 years nearer our time (AD 42 to 112) as redated above he could have observed the large total lunar eclipse of 65 Jan. 11 when he was about 23 years old, i.e. five years before his observations gained reputation.

Hero of Alexandria was a Greek mathematician and engineer living and working in Alexandria. Although some historians place his active period around BC 150, he has been "absolutely" dated to the middle of the first century AD because of one of his astonomical observations which appears to be "one of a kind": a lunar eclipse observed simultaneously in Alexandria and Rome. There is no year number specified, but the observation says 10 days before the vernal equinox and at 5th hour of the night at Alexandria. An investigation (ref 4) has shown that these circumstances were satisfied by only one lunar eclipse in the time span -200 to +300, namely that of AD 62 Mar. 13.

As mentioned above, some historians consider Hero to be contemporary with Hipparchos. Then this absolute dating of Hero to the middle of the first century AD makes our previously proposed redating of Hipparchos by 232 years towards our time reasonable!

Recently, a complete English translation of Geminos's "Introduction to the Phenomena" has been published (ref 5). Geminos wrote the "Introduction" as a textbook for beginning students of astronomy. The book is dated to the first century BC, that means the period between Hipparchos (as conventionally dated) and Ptolemy. Geminos mentions both Hipparchos and Hero as forerunners.

The following is a direct citation from one of the initial sections of the new publication:

"One minor problem with dating Geminos to the first century b.c. involves his mention of Hero of Alexandria in fragment 1. The dating of Hero has been controversial, with suggested dates from the middle of the second century b.c. to the middle of the third century a.d. .......  If Hero used an eclipse of recent memory, we must place him in the second half of the first century a.d. Thus, if the dating of Geminos to the first century b.c. is correct, we must suppose that Proklos or a later copyist interpolated the name of Hero in fragment 1."

There would not be any trouble at all, if Hipparchos and Hero were contemporary and lived in the second half of the first century AD, while Geminos lived in the second half of the second century AD!