Many planetary bodies experience tides, which produce time-varying stresses. Seismic activity on the Moon is modulated by tides, and there are hints of similar effects on Earth (but not, so far, Mars). In this talk I'll describe two other places where tides modulate geological activity at different periods: Io, a highly volcanic moon of Jupiter; and Enceladus, a small icy moon of Saturn. In both cases we can use remote-sensing observations of the modulation to make inferences about the properties of these bodies' interiors. One could imagine similar approaches being used for tidally-distorted exoplanets (e.g. the TRAPPIST system).

The enormous development of astrometry in the years since 1925 is the subject, astrometry being the branch of astronomy for high-accuracy measurement of stellar positions, distances and motions. -- As student of 21 years in 1953 I began to work at the new meridian circle at Brorfelde located 50 km from Copenhagen. I became fascinated by the instrument and saw the great importance of astrometry for astronomy and astrophysics. As a stipendiary in Hamburg, I proposed in 1960 a new method of astrometry by photon counting, using a photo-multiplier as detector and the then novel electronic computers for data reduction. The idea fitted well with plans for an expedition with the Hamburg meridian circle to Perth in Western Australia and we implemented the new method 1960-67 on the instrument. -- The new method was adopted by Pierre Lacroute in France in his design 1964-74 of a scanning satellite. Pierre Lacroute became the father of space astrometry. In 1975 ESA began a study of this idea. I became involved and have been able to contribute to space astrometry since then. The study led to the first satellite for space astrometry, Hipparcos, launched in 1989 on a three year mission and it became a great success. In 2013 a new astrometric satellite, Gaia, was launched by ESA and the first results have now shown that it is, as expected, a million times more powerful than Hipparcos. Plans for a Gaia successor with Near Infrared capabilities, GaiaNIR, for launch about 2035 have been worked out with partners in Europe, USA, Japan and Australia. We have proposed to ESA in Hobbs+2019 arXiv:1907.12535 that GaiaNIR be studied.