1. Radio Astronomy
  2. Data to Science
  3. Solar → Sidereal → RA

Data to Science · 1 of 3

Solar time → sidereal time → Right Ascension

How a fixed zenith telescope records the sky as the Earth rotates, and how each spectrum is placed on it.

The idea

A fixed telescope sees the sky go by

A zenith-pointing telescope never moves. As the Earth rotates, different parts of the sky pass overhead, so each recorded spectrum belongs to a different Right Ascension. The file knows only the clock time at which it was recorded. To place a spectrum on the sky, clock time must be turned into sidereal time, and sidereal time into Right Ascension.

The chain

Solar time → sidereal time → Right Ascension

  1. Clock / solar time (UTC)

    Each spectrum is stamped with a time. First record which time standard was used (UTC or local time) and its offset; mixing them silently shifts the sky position.

  2. Sidereal time

    Sidereal time follows the stars rather than the Sun. A sidereal day is about 23 h 56 m 4 s of solar time, so the stars rise about four minutes earlier each solar day. The reference value is Greenwich sidereal time.

  3. Local Sidereal Time (LST)

    Local Sidereal Time is the sidereal time at the telescope's longitude:

    LST = GST + east longitude

  4. Right Ascension

    For a telescope pointing at the zenith, the Right Ascension at the beam centre equals the Local Sidereal Time, and the declination stays close to the site's latitude:

    RA ≈ LST, Dec ≈ site latitude

    “Approximately” because the beam is a few degrees wide and the pointing is only as exact as the mount.

Original and derived

Keep the two apart

Original Recorded

The spectrum, its clock time stamp and the station's position. These are what the instrument and its logs recorded.

Derived Computed

Sidereal time, LST and Right Ascension. They depend on the time standard, the longitude and the pointing assumption, so they must be recorded as processing, not as measurement.

Real data

Historical RA / LST-corrected data

RA / LST explorer Awaiting ingestion

The historical fixed-telescope bundles that were already corrected to RA / LST (archive level A2) will be demonstrated here. They have not been ingested, so no interactive display is shown and no values are claimed.

See the PSU fixed archive lineage