1. Digital cameras, and the computers to analyze images.
2. Space telescopes (Hubble Space Telescope, the first large space telescope, only launched in 1990).
3. The building of massive ground-based telescopes. Before 1990, the largest telescope had a mirror diameter of 6 meters. Now, multiple 30-meter telescopes are under construction. Collecting power goes with the square of the diameter, so this is an increase of 25 times in collecting power!
4. Very recently, the development of gravitational-wave interferometers, which allow astronomers to observe a totally new type of radiation.
5. Advances in the multi-messenger observations, where apart of photons and gravitational waves, astronomers can detects also neutrinos with specialized neutrino detectors, e.g.: IceCube [a], though there are many more of those [b].
6. Advances in very-long-baseline interferometry [c] using a globe-sized array of radio-telescopes, like Event Horizon Telescope [d]
[a] https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory
[b] https://en.wikipedia.org/wiki/Neutrino_detector or here https://en.wikipedia.org/wiki/List_of_neutrino_experiments
[c] https://en.wikipedia.org/wiki/Very-long-baseline_interferome...
My clock certainly seems to tick in the opposite direction when I look at it from behind.
1. https://en.wikipedia.org/wiki/Cosmological_principle