Frequently Asked Questions

1. What is a celestial reference frame?

   A celestial reference frame (CRF) is a spherical coordinate grid overlaid on the sky in order to determine positions (and often motions) to celestial objects such as stars and galaxies. It is analogous to the grid of longitude and latitude used to determine positions on the Earth, but the coordinates are called right ascension and declination. Positions are either given in “sexagesimal” format: composed of hours, minutes, seconds of right ascension and degrees, arcminutes, arcseconds of declination (for example, R.A. = 04h30m49.059s, decl. = +64°50′52.60″), or they are given in decimal degrees (R.A. = 67.704412, decl. = 64.847944). The former is convenient for observers; the latter is convenient for numerical calculations. The CRF is the International Celestial Reference Frame (ICRF), which is now in its 3rd version. It is composed of 303 active galactic nuclei (AGNs)— supermassive black holes that actively accrete matter and emit radiation— selected at radio wavelengths where their positions can be precisely determined using very long baseline interferometry. These objects define the orientation of the ICRF, and an additional 4,233 radio-selected AGNs are included to improve the sky density of astrometric reference objects.

2. What is the relationship between the CRF and time?

   The U.S. Naval Observatory is responsible for precise timekeeping, and the nation’s clocks are kept synchronized with the USNO Master Clock, including those of computer systems and the Global Positioning System (GPS). The relationship between the CRF and timekeeping is complex, but in broad terms daily monitoring of the CRF allows for precise determination of the actual length of day (the time taken for the Earth to rotate once), which differs from the vastly more precise atomic time maintained by USNO. Once the difference between these two timescales has reached a certain tolerance—0.9 seconds—a leap-second is added to the official time to bring them back into agreement. Monitoring of the CRF is likewise required to precisely determine the orientation of the Earth, critical for modern position and navigation requirements.

3. What is astrometry?

   Astrometry is the mathematics and science of determining the positions and motions of celestial objects. Positions and motions are defined with respect to the CRF, specifically the ICRF, and are composed of six parameters: right ascension, declination, proper motion in both, parallax, and radial velocity. Proper motion is the amount by which right ascension and declination is changing over time. Parallax is the apparent change in the position a celestial object due to the Earth’s motion around the Sun, and is used to determine exact distances. The radial velocity captures a celestial object’s motion along our line of sight (as opposed to perpendicular to our line of sight in the case of its proper motion), and is typically estimated by comparing emission or absorption lines in the object’s spectrum to their laboratory wavelengths. Radial velocity towards us induces a blue-ward Doppler shift, while radial velocity away from us induces a red-ward Doppler shift. Celestial objects outside of the Solar System may seem static and unchanging. However, the increasing precision of astrometric measurements has revealed a universe in motion. Astrometry from the recent Gaia spacecraft has provided positions and motions for hundreds of millions of stars in our galaxy to unprecedented precision, allowing researchers to directly trace its history and formation over cosmic time. If technological advancements in astrometry continue to be made, the motions and histories of objects outside our galaxy may become accessible, allowing for direct exploration of a dynamic and changing universe.