Why it is time for the Moon

We understand Indian Standard Time (IST) and Greenwich Mean Time (GMT). Soon, we will have lunar standard time, also known as lunar coordinate time (TCL).

The National Aeronautics and Space Administration (NASA) has a lofty plan to return humans to the Moon as part of the ambitious Artemis mission. The Artemis I, the first uncrewed voyage to the Moon, was completed successfully in 2022. The craft orbited the Moon and crashed into the Pacific Ocean. The first crewed test flight, Artemis II, will be a lunar flyby scheduled for April 2026. Four astronauts will go around the Moon and return. Finally, astronauts onboard Artemis III will land on the Moon in mid-2027. A lunar base will thereafter be established, with permanent human habitation. China is also just a little behind, with ambitions to create its own lunar station.

Lunar base in the offing

Navigational and communication capabilities such as GPS and the internet will be necessary when a network of lunar bases is built. Precision time is an essential component of technologies like GPS and the internet. The passage of time varies across the cosmos, and it goes quicker on the Moon. As a result, the Moon will require its own clock to keep track of the ticks so that GPS, the internet, and other navigation and communication technologies work correctly.

In August 2024, the International Astronomical Union voted to adopt Lunar Coordinated Time (LTC), a timekeeping standard for the Moon.

Solar time to standard time

Just two hundred to three hundred years ago, life revolved around the Sun and the stars. Longer days denoted summer, whereas shorter days suggested winter. The shadow created by the gnomon revealed the time of day.

Of course, the Sun did not travel the same path in the sky since the axis tilted, causing the Sun's inclination to shift from south to north during the course of a year. This implies that a certain length of shadow did not correspond to a given hour every day of the year. Astronomers knew the complexities, but a simple gnomon was sufficient for ordinary people.

We know that daytime in India corresponds to nighttime in the United States. The Sun's position relative to the Earth's orientation determines the time at any particular location. On average, the Earth spins 360 degrees every 24 hours, which means it travels 15 degrees of longitude per hour, or one degree every four minutes. A one-degree variation in longitude translates to a four-minute time difference. This implies that at 6 a.m. in Allahabad (82°30' E), it will be roughly 6:36 a.m. in Guwahati (91° 42' 11'' E) and 5:21 a.m. in Ahmedabad (72° 35' 6'' E). Even between Chennai and Madurai, there is a time difference of 8 minutes and 16 seconds.

Time according to your longitude is known as 'solar time' since it was calculated using the gnomon's shadow or by witnessing the Sun transit the meridian. As a result, in the past, each kingdom and large town had its own time, which was determined by the position of the Sun and visible stars from that location.

This all changed with the introduction of trade and commerce, as well as new technology. By the mid-nineteenth century, telegraph lines were crisscrossing the Indian subcontinent, and railway networks were spreading to connect remote regions. Telegraphs needed time stamps to relay messages over the lines, and trains needed to coordinate their timetables. All of this required time synchronisation.

Railway time

The British founded the Madras Observatory in 1796, and in 1802, John Goldingham, an astronomer at the observatory, calculated its precise longitude and discovered that it was 5 hours, 21 minutes, and 14 seconds ahead of Greenwich Observatory, or the time in London. The British adopted this as their standard time in the Madras Presidency.

As colonialists strengthened their rule, each province established a standard time for official business based on its capital, such as Bombay Time (UTC+04:51), Calcutta Time (UTC+05:53:20), Madras Time (UTC+05:21:14), and Port Blair Mean Time (UTC+06:10:37). Because a single time framework was required for all India operations, the railroads and telegraphs adopted 'Madras time', which became known as 'railway time' in common perception. Finally, worldwide efforts established conventional time zones in sync with Greenwich. The Indian Standard Time was adjusted to 82.5E, or + 5:50 ahead of GMT. With GMT and time zones, worldwide trade, commerce, shipping, and, eventually, aviation traffic could be organised.

Wobbling of the Earth

The 2004 Sumatra earthquake and tsunami caused the Earth to revolve faster, resulting in shorter days. The Earthquake affects the Earth's shape, and the oblateness (flattening on top and bulging at the equator) can reduce or increase by a tiny amount. Just as a spinning ballet dancer draws her arms closer to her body for a quicker spin and stretches her hands for a slower spin, the lean Earth spins faster, making the days shorter. In contrast, the fatter Earth spins slower, making the days longer. Scientists determined that the 2004 tsunami lowered the duration of the day by 2.68 microseconds. A microsecond is one-millionth of a second. They are so tiny that until recently, we had no way of measuring them.

Many variables, including earthquakes, influence the rate of Earth's rotation. Remember that solar time is tied to the rotation of the Earth. As a result, if the clock runs continuously 24 hours a day, it will eventually go out of sync with solar time.

The precise duration of the year is 365.2422 days, not 365. Just as we add an extra day, February 29, every four years to maintain the 365-day calendar synchronised, a leap second is added every now and then to keep the clock in sync with the Earth's rotation. The practice began in 1972, and till today, 27 leap seconds have been added, the latest recent on December 31, 2016.

From Telegraphs to GPS

Railways required synchronised schedules, and telegraphs called for time stamps for relay transmission, prompting the adoption of GMT as the global clock time. The 1980s witnessed the rise of digital technology, which created a new demand for timekeeping.

When you hit the ' send' button on your email server, the message is divided into bits and pieces, known as packets. Each packet is timestamped sequentially, along with the sender and reviver addresses. Each packet takes its own course via the internet and arrives at the receiving end mixed up. The device at the receiving end reads the time stamp and reassembles the packets to restore the message's original sequence. Even minor errors in the time stamp will result in gibberish.

The GPS, too, is dependent on precise timekeeping. At least three GPS satellites ping the device, and the time it takes for the signals to return is used to estimate the exact location of the device on the Earth's surface. Mobile cell networks, too, bank on precise time stamps.

These technologies require more precise time reckoning than the Earth's irregular rotation allows, leading to the development of atomic clocks. Atomic clocks monitor the passage of time by using the natural vibrations of atoms rather than temperature-sensitive components such as pendulums, springs, or uneven oscillations such as Earth's rotation. This resulted in the notion of Universal Coordinate Time, or UTC, based on around 450 atomic clocks maintained in 85 national time labs worldwide with an astounding precision of one-tenth of a billionth of a second daily.

Although UTC is appropriate for terrestrial requirements, the space age needs an alternative way of timekeeping. If and when the lunar base is completed and people begin to settle, they will suffer greatly if they just adopt UTC.

We must first address the fundamental question of 'what is time' to comprehend why this is so.

The flow of time

The rapid roar of a river in steep gorges becomes a tranquil flow on the smooth slop; similarly, Einstein stated that the flow of time is not absolute across space and time. According to the Special Theory of Relativity, motion has a direct effect on the flow of time, which means that the faster an object moves relative to an observer, the slower time appears to pass for that object, a phenomenon known as "time dilation"; essentially, time is relative to the observer's frame of reference, and high speeds cause time to "slow down" for the moving object. Atomic vibrations on a moving object appear to take longer to complete one oscillation when compared to a 'stationary' observer.

Because the Moon is moving relative to Earth, this motion will affect how the clocks on the Moon tick compared to those on Earth. In 1915, Einstein proposed the General Theory of Relativity, which describes what happens to the flow of time near a gravitational potential. According to this theory, time moves slower in a strong gravitational field. Hence, a clock near a massive object like Earth will tick slower than a clock on the Moon, which is less massive.

“More than 50 years ago, humans landed on the Moon, but we didn't need any of these things. We could just relay communications. But if you want to have a continuous presence and build a colony for exploration we need to address this issue," says Bijunath Patla of the National Institute of Standards and Technology, Colorado, who had teamed up with his college Neil Ashby to recently crack the problem of estimating the difference between the flow of time on Earth and the Moon.

It's complicated

How much will the clocks on Earth and the Moon differ owing to relativistic effects? Calculating it is messy. To begin, two variables complicate things: one makes the clock tick slower, and the other causes it to beat quicker on the Moon than on Earth. Tidal forces and Earth's gravitational pull all influence the flow of time on the Moon. Properly dealing with relativity's consequences necessitates selecting a suitable frame of reference, something astronomers have long struggled with.

Neil Ashby and Bijunath Patla devised a fascinating method to calculate time dilation. A satellite circling the Earth is in free fall, which means it always falls towards the Earth. However, due to the Earth's curved surface, it misses and continues to circle. Similarly, they argued we might picture the Earth-Moon system orbiting the Sun in free fall.

Considering the Earth-Moon system orbiting their mutual centre of mass while in free fall under the influence of the Sun's gravity allowed them to better formulate the contributions of each complicated factor, such as deviation from the even distribution of mass around Earth and Moon, mutual tidal forces, rotation of each object around its axis, and so on. They discovered that the drift was 56.02 microseconds per day (0.0000562 seconds).

“Before, there were different calculations of lunar time, ranging from 56 to 58 microseconds faster per day. Now, we know that it is 56.02 microseconds per day. If we were to navigate with this accuracy, we would be within 10 meters of error," says Patla.

Why it matters?

Although a 56.2 microsecond per day difference is little by human standards, it is substantial in current digital navigation and communication technology. Light moving at over 3 lakh kilometres per second would have covered around 17 km. A mere 56.2 microsecond per day difference might result in navigational mistakes as great as 17 miles per day. To ensure a precise landing and seamless operation on the Moon, astronauts, rovers, bases, and landing sites must be pinpointed within 10 meters at all times. If time dilation is not considered, data transmission between Earth and Moon over the internet may fail.

“If you send communications, observers on the Moon and on Earth are off by a few microseconds. And it keeps adding over time and becomes a huge time difference. If we are to land a spacecraft on the Moon precisely, within meters, then we need to know the common time between Earth and the Moon,” says Patla.

Placing clocks on the Moon

An agreed-upon standard lunar time reference will ensure the technical synchronisation of lunar-based navigation and communication systems and the appropriate execution of scientific activities on the lunar surface. A network of clocks must be established on the Moon's surface and in Lagrangian points where the gravitational pulls of Earth and Moon are balanced to coordinate lunar time for use by all space-faring nations.

“The clocks placed on the could be as small as a bedside alarm clock, but I would say with the electronics and everything, they should be around the size of a foot in each direction. Somewhere close to the equator of the Moon is an ideal location for a reference clock,” says PaWea and added, “We actually slow those clocks down by that 56.02 microseconds amount while it is still on Earth, and then launch it so that when it is on the Moon, it has the same time as a clock on the surface of the Earth.”

What about beyond the Moon? Mars time, Saturn time? When asked, Patla replied, “That is the next project. We wanted to make the building blocks very transparent and use the theory in a fundamental manner.”