The University of Texas at Arlington

The University of Texas at Arlington

UTA Planetarium

UTA Planetarium

Ask the Astronomer Q&A

Tag: "ecliptic"

  • Does the longitude of the Vernal Equinox move around the equator once during 25,800 cycle of the precession of the equinox?
  • Yes, the vernal equinox does change its position due to precession. The equinox points are the points in space where the celestial equator and ecliptic meet. The one that the Sun travels from Southern hemisphere to Northern hemisphere is called Vernal point. This intersection point moves Westward in space at approximately 50 arcsecs/year (Precession motion). That’s why NCP moves across the sky changing the North Star in the long-term. One cycle takes approximately 26,000 yrs. Precession motion changes the position of Vernal Point in reference to the background stars. But the Vernal Point will always be Right Ascension (RA) = 0 because we defined it that way. This means that RA and Declination (DEC) of stars will shift over a long time. For this reason, star catalogues include the year when listing RA and DEC of stars.

    (Tags:  ecliptic  equinox  precession)
  • Does the plane of revolution of planets have anything to do with the angle of which their parent star rotates?
  • The revolution of all planets are in a plane called the invariable plane, that is defined with the angular momentum of the solar system. But there is an uncertainty on the invariable plane as we don’t know every object in the solar system. For this reason, we call ecliptic, the plane Earth revolves around the Sun, the plane of the Solar System for convenience. Sun’s rotation axis is tilted by about 7 degrees to the perpendicular to the ecliptic. Stars’ rotation axis has a lot to do with the invariable plane during the formation of stars, but we can’t rule that the invariable planes will be in 90 degrees tilt with rotation axis of host stars.

    (Tags:  ecliptic  solar system  star  sun)