The crossword solution for the term 'ONE WHO KNOWS WHAT'S UP?' is 'ASTRONOMER' (10 letters). An ASTRONOMER is a scientist who studies celestial objects, such as stars, planets, comets, and galaxies. Astronomers use telescopes and other instruments to observe and measure the properties of these objects, and use mathematics and physics to explain their observations. They also use their knowledge to make predictions about the future behavior of celestial objects.