The Distant Sun

Growing up in a village about 30 years ago, I used to hear that the Sun was a large sphere of burning coal that had been ablaze for a long time. I accepted this as fact until I reached 9th and 10th grade, where I learned from my textbooks about nuclear fusion—the true source of the Sun's vast energy. This was when I realized that the old model of the Sun as a coal-burning sphere had been discarded.

Interestingly, those same textbooks mentioned that the coal model was once a widely accepted belief and a proposed explanation for the Sun's energy. However, with advancements in science, this model was eventually disproven and abandoned.

I am also grateful to that old man from my childhood who, despite the misconceptions, didn't feed me any mythological explanations about the Sun.

That sets the stage for what I'll be discussing next. I'll be providing a brief overview of how the distance between the Earth and the Sun has been calculated, along with the significant contributions made by scientists in this area.

Before diving into that, let's discuss the concept of units. A unit is a standard measurement that defines how a quantity is assessed or expressed. It provides a basis for quantifying and comparing different measurements.

For example, when measuring length, units like meters, feet, or miles are used. If you measure a distance as 5 meters, the unit "meters" indicates the standard length being used to quantify that distance. Similarly, if a distance is given as 3 miles, the unit "miles" provides the context for the measurement.

Yojana

Here, it's important to clarify that when we talk about distance, we measure it in specific units. There are plenty of units used around the world—feet, meters, yards, and so on. In ancient India, units like yojana[^1] and kosa were also used to measure distance.

Earth-Sun Distance

The distance between the Sun and Earth is approximately 151.62 million kilometers. In reality, this distance varies slightly every moment due to Earth's elliptical orbit. It ranges from about 147 million kilometers to 152 million kilometers. Interestingly, this variation in distance doesn't affect the seasons—whether it's summer or winter is determined by Earth's axial tilt, not its proximity to the Sun.

Historical Estimates of the Earth-Sun Distance by Different Scientists

1. Aristarchus of Samos (3rd century BCE)

• Method: Geometric calculations based on the relative sizes and distances of the Earth, Moon, and Sun, using the phases of the Moon.
• Estimated Distance: About 20 times the Earth-Moon distance (actual ratio is about 400 times).
• Accuracy: Highly inaccurate; underestimated by a factor of around 20.

2. Johannes Kepler (1609)

• Method: Kepler's laws of planetary motion provided the relative distances of planets from the Sun, but not an absolute distance.
• Estimated Distance: Kepler didn't provide a precise kilometer value, only relative distances in astronomical units (AU).
• Accuracy: Accurate in terms of relative distances but not in absolute distance.

3. Jean-Dominique Cassini (1672)

• Method: Parallax method using observations of Mars from different locations on Earth (in Paris and French Guiana) to determine its distance and then applying Keplers laws to estimate the Earth-Sun distance.
• Estimated Distance: Approximately 140 million kilometers.
• Accuracy: Fairly accurate, off by about 10 million kilometers.

4. Transits of Venus Observations (1761 & 1769)

• Method: Parallax method during the transits of Venus across the Sun, where observations from different locations on Earth were used to calculate the Earth-Sun distance.
• Estimated Distance: Around 153 million kilometers.
• Accuracy: Very close to the modern value, off by about 3 million kilometers.

5. Radar Measurements (1940s-1950s)

• Method: Radar waves were bounced off planets like Venus, and the time taken for the signals to return was measured. Kepler's laws were then used to determine the Earth-Sun distance.
• Estimated Distance: 149.6 million kilometers.
• Accuracy: Extremely accurate, within a few hundred kilometers of the modern value.

6. Modern Value (21st Century)

• Method: Defined using the speed of light and the time it takes for light to travel from the Sun to Earth, along with highly precise measurements from spacecraft and Very Long Baseline Interferometry (VLBI).
• Exact Distance: 149,597,870.7 kilometers.
• Accuracy: This is the officially defined astronomical unit (AU) and is exact by definition.

In modern times, the distance between the Earth and the Sun is calculated with remarkable precision. Advances in technology allow for exact measurements at any given moment, accounting for the elliptical nature of Earth's orbit. The value of 149,597,870.7 kilometers for the astronomical unit represents the average distance over the course of a year.

Scientific methods involve both theoretical frameworks and practical calculations that can be independently verified. This ensures that the results are reliable and consistent across different observations and experiments.

Now, let's address the incorrect claim:

Some people claim that Tulsidas predicted the distance from the Earth to the Sun in his Hanuman Chalisa. Specifically, they refer to the line:

Yuga sahastra yojana par bhanu,
Liliyo tahi madhur phal janu.

This translates to:

"Lord Hanuman swallowed the Sun, which was located at a great distance (for a long time for yug possibly)."

This interpretation suggests that Tulsidas had some knowledge of the distance between the Earth and the Sun. However, setting aside the mythological aspects, let's focus on why the distance claim is flawed.

The claim is that the distance described as "yuga-sahastra-yojana" equals 153.6 million kilometers. According to their calculations:

Earth-Sun Distance (d) = yuga × sahastra × yojana

Where:

• yuga = 12,000 years
• sahastra = 1,000
• yojana = 8 miles
• 1 mile = 1.60934 kilometers

When you multiply these values, they arrive at 153.6 million kilometers.

But here's the issue: The unit of this calculation is years-kilometers.

This means the calculation is combining units of time (years) with units of distance (kilometers), which doesn't make sense for measuring distance. Thus, the calculation is fundamentally flawed.

So, the claim is not accurate. It is more plausible that Tulsidas was metaphorically expressing that the Sun stays at a great distance (measured in thousands of yojanas) for a long duration (across yugas), rather than providing a literal measurement of the Earth-Sun distance.

Tulsidas was not an ancient scientist, mathematician, or astronomer like Aryabhata or Bhaskara. He was a poet of his time, known for his significant contributions to poetry and epic literature. Although certain aspects of his work may be subject to criticism, his impact on literature and poetry remains substantial and noteworthy.

[^1]: The value of a yojana, when converted to SI units, is estimated to be approximately 13.54 kilometers. This estimation is based on Āryabhaṭa's calculations of the Earth's circumference and other historical measurements