APOD: 2020 September 19 - Orion in Depth

Good morning, Rallypoint. Today's Astronomy Picture of the Day (APOD) is titled "Orion in Depth." Many here have commented on Orion as their favorite or most easily recognized constellation. Yet our perspective of Orion from Earth is only two-dimensional. What if we could see Orion in three dimensions?

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. In astronomy, the term parallax is the semi-angle of inclination between two sight-lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit.

Launched in 1989, the European Space Agency's (ESA) Hipparcos (HIgh Precision PARallax COllecting Satellite) used the principles of parallax to complete its mission as the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky. It is from the Hipparcos data sets that we have our three-dimensional view of Orion. Would you have guessed that the middle star in Orion's belt (Alnilam) is the one furthest away from Earth? The name Hipparcos is also a hat tip to the Greek astronomer Hipparchus of Nicaea (190-120 BC), who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.