Time Seeds of Universe -1
Three years ago when life was imminently ephemeral, a hibernated desire to understand time scale of universe sprouted and started to grow. Life during those months of lockdown were full of anxiety and uncertainty. On the human’s time scale, I started on a journey to explore the universal time. To my own surprise, I could find an hour in the morning to study despite of pandemic induced cooking, household chores, a job and an inquisitive child at home. My exploration was all zig-zag with internet as the key source, referring to multiple sites at a time. Since I didn’t have a structure or a course, I listed down several questions for myself and started to search for their answers.
- How did human start tracking time?
- What were the methods and instruments ?
- What is basis for Ancient Civilisations’s calendar ?
- What is Solar, Lunar and Luni-Solar calendar systems?
- What was Ancient Hindu’s Calendar System?
Each of these questions led me to more questions and a complex maze of astronomy, history, archaeology, sociology, mindfulness and consciousness. Everyday morning, I would wake up to learn new definitions, meaning and awareness of the world around me. I was not well-equipped to understand a lot of the concepts because human history of time tracking is intrinsically linked to our understanding of celestial bodies, patterns of movement and various assumptions made at different phases. Before I could start the journey, I had to prepare with essentials of today’s astronomy to go back in time.
Earth’s orbit around the Sun
I started with fundaments – the high- school concepts of solar system. Earth rotates, ie., spins about its own axis in 24 hours. Earth revolves, has an orbital motion around Sun in 365 days. And a satellite revolves around a planet; our only moon takes 29.5 days to complete the revolution around earth.
Earth’s orbit around the Sun is an approximate ellipse and not a circle. But the more interesting part I had not studied in the school was the two body problem. Both the bodies revolve around the same focus in ellipse, the one closer to the more massive body. But when one body is significantly more massive, such as the Sun with respect to the Earth, the focus may be contained within the body of the larger mass. Thus the smaller body or the secondary body, Earth, is said to revolve around the larger body, the primary body. In an elliptical orbit, there are two foci or apsides– the farthest and the nearest point in the orbit of a planetary body. The line of apsides is the line connecting these two extreme values. For orbits about the Sun, the apsides are called aphelion (farthest) and perihelion (nearest). As if the two body problem is not interesting enough, there is a multi-body problem with a common centre of mass called the barycentre. Here, the actual motion of a body is perturbed due to the additional gravitational effects of the remaining body or bodies. If there is only one other significant body, then the perturbed motion is a three-body problem; if there are multiple other bodies, it is an n-body problem.
Imagine a system with multiple objects with different masses having different gravitational forces, affecting each other’s motion. Thankfully, for our solar system, the Sun is massive enough to be the barycenter of the solar system and make all other planets revolve around itself. The revolution or orbit of all the planets are nearly elliptical path. For an observer outside the solar system, (or with reference to the stars) Earth revolves in its orbit around the Sun in 365 days, 6 hours, 9 minutes, at a speed ranging from 29.29 to 30.29 km/s. Sun, too has its own path in the Milky Way, at a speed of 230km/s but its speed and path does not seems to be relevant for general human life on earth. We take the Sun granted for being there, always. Not that we think so much about Earth’s revolution but we see the Sun rising and setting that has kindled human’s curiosity to dig into this phenomenon.
Similar to earth’s orbit around Sun, the Moon’s path around the Earth is also elliptical. Moon revolves around earth in its orbit at a speed of 1.02km/s and takes about 27 days to complete the revolution. The point in the Moon’s orbit that is closest to the Earth is called the perigee and the point farthest from the Earth is known as the apogee.
Earth reaches its closest approach to the Sun, a perihelion of 147,090,000 km, on about January fourth of each year (about two weeks after the December Solstice). Aphelion comes six months later at 152,100,000 km in July (about two weeks after the June Solstice). Due to variations in the eccentricity of the Earth’s orbit, the dates when the Earth reaches its perihelion or aphelion are not fixed in one solar cycle. This path’s shape also varies due to the gravitational influences of other planetary objects.
The distance of Earth from the Sun and its timing appear a bit confusing because mind tends to assume that Earth should be at the farthest end from the Sun in December and closest to it in June. But what appears may not always be the truth!
I keep on looking at this beautiful Eyes on the Solar System created by NASA. With a click of a mouse, we can zoom in to the depth of Sun and zoom out of the Milky Way galaxy.
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Time Seeds of Universe is a series on understanding time. ‘Time Seeds of Universe : A Journey of Self-Discovery’ is part 1 in this series. Following are the link to other blogs of this series :
Time Seeds of Universe – 2: It is all about Reference Frame
Time Seeds of Universe – 3 : A Sphere with Infinite Radius
Time Seeds of Universe- 4: Path, Light and Shadows
Time Seeds of Universe – 5: Quest for Salvation in the Wonders of Nature
Image Credit:
Hiranyagarbha. The Cosmic Egg. Manaku of Guler. 1740. Bharat Kala Bhavan, Varanasi.
