Radar systems measure the distance to a target by the time it takes a radio-wave pulse to return to the radar antenna after being reflected by the target: the distance to the target is half the round-trip transit time multiplied by the speed of light. A Global Positioning System (GPS) receiver measures its distance to GPS satellites based on how long it takes for a radio signal to arrive from each satellite, and from these distances calculates the receiver's position. Because light travels about 300000 kilometres (186000 mi) in one second, these measurements of small fractions of a second must be very precise. The Lunar Laser Ranging Experiment, radar astronomy and the Deep Space Network determine distances to the Moon, planets and spacecraft, respectively, by measuring round-trip transit times.
Interferometry is another method to find the wavelength of electromagnetic radiation for determining the speed of light. A coherent beam of light (e.g. from a laser), with a known frequency (f), is split to follow two paths and then recombined. By adjusting the path length while observing the interference pattern and carefully measuring the change in path length, the wavelength of the light () can be determined. The speed of light is then calculated using the equation c = f.
Before the advent of laser technology, coherent radio sources were used for interferometry measurements of the speed of light. However interferometric determination of wavelength becomes less precise with wavelength and the experiments were thus limited in precision by the long wavelength (~0.4 cm) of the radiowaves. The precision can be improved by using light with a shorter wavelength, but then it becomes difficult to directly measure the frequency of the light. One way around this problem is to start with a low frequency signal of which the frequency can be precisely measured, and from this signal progressively synthesize higher frequency signals whose frequency can then be linked to the original signal. A laser can then be locked to the frequency, and its wavelength can be determined using interferometry. This technique was due to a group at the National Bureau of Standards (NBS) (which later became NIST). They used it in 1972 to measure the speed of light in vacuum with a fractional uncertainty of 3.5109.
Before October 2014, the settlement of South Sulawesi by modern humans had been dated to c. 30,000 BC on the basis of radiocarbon dates obtained from rock shelters in Maros. No earlier evidence of human occupation had at that point been found, but the island almost certainly formed part of the land bridge used for the settlement of Australia and New Guinea by at least 40,000 BCE. There is no evidence of Homo erectus having reached Sulawesi; crude stone tools first discovered in 1947 on the right bank of the Walennae River at Berru, Indonesia, which were thought to date to the Pleistocene on the basis of their association with vertebrate fossils, are now thought to date to perhaps 50,000 BC.
Following Peter Bellwood's model of a southward migration of Austronesian-speaking farmers (AN), radiocarbon dates from caves in Maros suggest a date in the mid-second millennium BC for the arrival of a group from east Borneo speaking a Proto-South Sulawesi language (PSS). Initial settlement was probably around the mouth of the Sa'dan river, on the northwest coast of the peninsula, although the south coast has also been suggested. Subsequent migrations across the mountainous landscape resulted in the geographical isolation of PSS speakers and the evolution of their languages into the eight families of the South Sulawesi language group. If each group can be said to have a homeland, that of the Bugis today the most numerous group was around lakes Tmp and Sidnrng in the Walenna depression. Here for some 2,000 years lived the linguistic group that would become the modern Bugis; the archaic name of this group (which is preserved in other local languages) was Ugiq. Despite the fact that today they are closely linked with the Makasar, the closest linguistic neighbours of the Bugis are the Toraja.
Pre-1200 Bugis society was most likely organised into chiefdoms. Some anthropologists have speculated these chiefdoms would have warred and, in times of peace, exchanged women with each other. Further they have speculated that personal security would have been negligible, and head-hunting an established cultural practice. The political economy would have been a mixture of hunting and gathering and swidden or shifting agriculture. Speculative planting of wet rice may have taken place along the margins of the lakes and rivers.
In Central Sulawesi there are over 400 granite megaliths, which various archaeological studies have dated to be from 3000 BC to AD 1300. They vary in size from a few centimetres to around 4.5 metres (15 ft). The original purpose of the megaliths is unknown. About 30 of the megaliths represent human forms. Other megaliths are in form of large pots (Kalamba) and stone plates (Tutu'na).
In October 2014 it was announced that cave paintings in Maros had been dated as being about 40,000 years old. Dr Maxime Aubert, of Griffith University in Queensland, Australia, said that the minimum age for the outline of a hand was 39,900 years old, which made it "the oldest hand stencil in the world" and added, "Next to it is a pig that has a minimum age of 35,400 years old, and this is one of the oldest figurative depictions in the world, if not the oldest one."