Positions & Motions of Stars

The purpose of SIM PlanetQuest is to provide the most precise astrometric measurements ever. Astrometry, the oldest branch of astronomy, is the science of measuring positions, distances, and motions of celestial objects. Any object such as a star can be characterized by five fundamental astrometric parameters:

RA and Dec: Position in the Sky

Right ascension and declination are the coordinates used to identify the position of an astronomical object. It is helpful to imagine these coordinates with respect to an imaginary "sphere" surrouding the Earth called the Celestial sphere. The Celestial equator, North Celestial Pole, and South Celestial Pole are equivalent to the Earth's equator and poles projected onto the Celestial sphere.

Right ascension (RA) and declination (Dec) are similar to latitude and longitude on the surface of the Earth. However, while latitude and longitude are fixed with respect to positions on the Earth, RA and Dec are fixed with respect to positions of stars on the sky. Declination (like latitude) is measured in degrees north or south of the Celestial equator. Right ascension (like longitude) is measured in units of hours, minutes, and seconds eastward from the position of the vernal equinox on the Celestial equator. The Vernal Equinox is the position of the Sun on the first day of Spring.

Proper Motion: Long Term Change in Position

Parallax is only the apparent motion of a star with respect to the background stars. Stars are actually moving at high velocities through space. The stars are so far away that this motion, known as proper motion, can be difficult to detect. Proper motion is the motion of the star perpendicular to our line of sight, or "sideways" in the sky (as opposed to moving towards or away from us). Proper motion (denoted by ) is expressed as an angular velocity, or the number of arcseconds per year the star travels.

Java Applet: See how the constellations change over time due to proper motion.

SIM PlanetQuest will measure proper motions of stars to an accuracy of 2 microarcseconds per year, where 1 microarcsec = 10-6 arcsec.

Parallax: Annual Cyclical Motion

As the Earth orbits the Sun, the position of nearby stars seems to change with respect to the background stars. For example, a nearby star (shown in pink in the diagram below) appears to be in a different position in the sky relative to the background stars (blue in the diagram) than it was six months earlier.

Java Applet: See how parallax angle changes with distance to a star.

Astronomers can measure the angle (p) of displacement of the star, and then use simple geometry to determine the distance (d) to the star, given that the distance from the Earth to the Sun is one Astronomical Unit (AU), where 1 AU = 1.5 x 10 11 meters. The distance is given by:

The parallax angle (p) is measured in units of arcseconds (arcsec), where 60 arcsec = 1 arcminute (arcmin), and 60 arcmin = 1 degree.

The distance (d) is measured in units of parsecs (pc), where 1 pc = 3.086 x 1016 meters.

SIM PlanetQuest will use optical interferometry to measure the parallaxes of hundreds (thousands?) of stars throughout the Galaxy. Interferometry is the process of combining light from two or more telescopes as if they were pieces of a single, gigantic telescope mirror. Click here for more information on interferometry. SIM PlanetQuest will be able to obtain an accuracy of 4 microarcseconds in parallax measurements, where 1 microarcsec = 10-6 arcsec.

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