23 September 1846 The planet Neptune is discovered by mathematical predictions. Too dim to be visible to the naked eye the first observations of Neptune were possible only after the invention of the telescope. The existence
of the planet was determined at the same time, but independently, by French astronomer Urbain Le Verrier and English astronomer John Couch Adams leading to an international dispute over discovery. Nevertheless, the Royal Society awarded Le Verrier the Copley medal in 1846 for his achievement,
without mention of Adams.
2 September 1988
The Australia Telescope Compact Array is opened. Situated about 25 km west of Narrabri, NSW, it is an array of six identical 22-metre diameter dishes. The Compact Array is a part of the Australia Telescope National Facility network of radio telescope and is used for radio astronomy.
14 September 2015
The first gravitational waves are discovered by the LIGO detectors in the USA. Prior to this, gravitational waves had only been inferred indirectly. Efforts to directly prove the existence of such waves had been ongoing for over fifty years, and the waves are so minuscule that Albert Einstein himself doubted that they could ever be detected.
7 September 1914
James Van Allen. American space scientist. He was instrumental in establishing the field of
magnetospheric research in space which led to the discovery of belts of trapped radiation above the
Earth's atmosphere which were named the Van Allen radiation belts in his honour. Diagram above courtesy NASA.
14 September 1915 John Dobson. American amateur astronomer famous for the Dobsonian telescope. He was well known for his efforts to promote awareness of astronomy through his "sidewalk astronomy." and was
the co-founder of the amateur astronomical group, the San Francisco Sidewalk Astronomers. He visited Sydney Observatory several times and gave a talk in 2003. Here is a link which describes his visit.
29 September 1954 Geoffrey Marcy. American astronomer. He is one of the pioneers and leaders in the discovery of exoplanets. Marcy and his research teams are recognised for discovering 70 out of the first 100 known exoplanets.