Practical impact on astronomy — The future of Coordinated Universal Time

Dennis McCarthy, International Astronomical Union Representative to the Consultative Committee for Time and Frequency and BIPM

Astronomical applications that use the current definition of Coordinated Universal Time (UTC) to access UT1 — the Universal Time based on Earth’s rotation — would be impacted by any change in the definition of UTC.

The rotation angle of the Earth in a celestial reference system is described by the angle UT1. The current definition of UTC ensures that the difference between UT1 and UTC (i.e., UT1‑UTC) remains under 0.9 seconds, which allows easy access to UT1 through UTC for those applications that do not need high precision.

If UTC is redefined, applications that currently use UTC as a low-accuracy representation of UT1 may be compelled to change their strategy, update their basic software, and educate their users about these changes. Any change in the definition of UTC could also present a cause for concern among producers and users of astronomical data.

These applications include:

• terrestrial telescopes, antennas, and other instruments pointed in precise directions;
• astronomical software and applications that assume the current definition of UTC;
• astronomical data in almanacs and websites employing the current definition of UTC; and
• the provision of observed or predicted values and parameters to describe the orientation of the Earth with respect to astronomical reference systems.

Pointing of terrestrial telescopes, antennas, and instruments

Astronomical pointing applications that require UT1 with better precision now obtain current and predicted estimates of UT1‑UTC from the Internet, GPS or BeiDou to satisfy their needs.

However, the current definition of UTC does not allow for UT1‑UTC values to differ by more than nine-tenths of a second. If this changes, much existing software may have to be modified to allow for more digits in UT1‑UTC.

Even so, if the current operational needs of telescopes for UT1 are met with the current accuracy of UTC, then the same accuracy requirement would presumably be satisfied within ±1 second in UT1‑UTC, in which case software adjustments might be minimal.

Such issues are expected to be resolvable with adequate lead time in the implementation of a new definition of UT1‑UTC.

Astronomical software and applications

Similarly, any existing software and applications that assume UTC is essentially equivalent to UT1 may require modification, allowing for corrections to include values of UT1‑UTC. Current and predicted approximations of UT1‑UTC from the Internet, as well as from GPS or BeiDou, are expected to satisfy this need, but their software and procedures might require modification.

Again, these issues can be resolved with adequate lead time.

Astronomical data in almanacs and websites

Ephemerides (predictive tables of planet, comet, or satellite positions) and predictions of astronomical phenomena are computed using a continuous time scale independent of the Earth’s rotation. Consequently, these calculations would be unaffected by any change in the definition of UTC. However, the resulting information may rely on UTC as a reference time.

Adjustments to UTC are unpredictable and are announced only months before their implementation. Astronomical ephemerides for future dates over longer-time spans could become erroneous due to unanticipated UTC adjustments along the way.

Ephemerides expressed in UTC would consequently benefit from removing the leap-second adjustments that contribute to the prediction error in UT1‑UTC. In fact, the possibility of this error would be eliminated.

Providing observed and predicted values of UT1‑UTC

The International Earth Rotation and Reference System Service (IERS), established by the International Astronomical Union (IAU) and the International Union of Geodesy and Geophysics (IUGG), is responsible for providing observed and predicted values of UT1‑UTC. In that capacity, it is responsible for announcing the one-second adjustments to UTC known as “leap seconds.”

Proposed changes in the definition of UTC might allow for a difference larger than 0.9 seconds between UT1 and UTC.

The role of the IERS, currently focused on announcing leap seconds, would then change. However, a proposed redefinition of UTC would likely increase the importance of IERS activities, with a renewed focus on providing UT1‑UTC data, perhaps even on a real-time basis.

In summary, the redefinition of UTC would certainly have an impact on astronomical efforts, but sufficient lead time would allow the astronomical community to adjust to that impact with no disruption in current procedures.

This article first appeared in ITU News Magazine: The future of Coordinated Universal Time – part of a series of editions on topics to be discussed at the World Radiocommunication Conference (WRC-23), from 20 November to 15 December in Dubai, UAE.

Download your copy of the ITU News Magazine: The future of Coordinated Universal Time.

Header image credit: Adobe Stock/NASA