                        Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                              2025 September 13

                  Star Trails over One-Mile Radio Telescope
             Image Credit & Copyright: Joao Yordanov Serralheiro

   Explanation: The steerable 60 foot diameter dish antenna of the
   One-Mile Telescope at Mullard Radio Astronomy Observatory, Cambridge,
   UK, is pointing skyward in this evocative night-skyscape. To capture
   the dramatic scene, consecutive 30 second exposures were recorded over
   a period of 90 minutes. Combined, the exposures reveal a background of
   gracefully arcing star trails that reflect planet Earth's daily
   rotation on its axis. The North Celestial Pole, the extension of
   Earth's axis of rotation into space, points near Polaris, the North
   Star. That's the bright star that creates the short trail near the
   center of the concentric arcs. But the historic One-Mile Telescope
   array also relied on planet Earth's rotation to operate. Exploring the
   universe at radio wavelengths, it was the first radio telescope to use
   Earth-rotation aperture synthesis. That technique uses the rotation of
   the Earth to change the relative orientation of the telescope array and
   celestial radio sources to create radio maps of the sky at a resolution
   better than that of the human eye.

                     Tomorrow's picture: tilts and spins
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
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