The Chandler Wobble

Earth precesses around its axis every ~26000 years.


But in addition to this precession, there is an extra wobble that was observed by Kustner and later followed up by Seth Carlo Chandler, Jr called the Chandler Wobble that occurs at a much smaller time scale.

In 1888, Kustner found that the latitude of Berlin had changed slightly
during his observations of the night sky.

Therefore in 1891,Chandler. decided to conduct a 14 month study examining this change. The following is a plot of the spiral path taken by the earth’s axis over that 14 month  period.


The following plot shows the motion from 1909 – 2001.



Although many theories indicate that this is due to the fact that earth is not a perfect spherical rigid body, it is still not entirely clear on the mechanism that drives earth into this small wobbly motion.

If you took a closer look at the plots you would find that this wobble is of the order of a couple of meters which most certainly does not seem like a lot.

But if you are an astronomer if you do not account for this correction, you might just end up pointing your telescope at the wrong object

Have a great day!

One of the striking aspects of our solar system is that the orbital plane of all the planets are similar i.e Its like the following:



And not like so:



But if you are puny human sitting on earth, how would one visualize this ? It’s easy!

Step out and look at the trajectory taken by the  sun and planets in the sky:




You will notice that the trajectories taken by the sun and the planets are similar in the night sky.

This gives you a visual validation of the fact that the orbital plane of all the planets and the sun are similar. Just a little something that you may or may not have realized about the cosmos.

Go ahead, give it a shot and have fun!

* The ecliptic plane is the name given to the mean plane in the sky that the Sun follows over the course of a year; 

Are the heavens moving or are we?


In the previous series of posts, we discussed about the key role that a rotating earth plays in space shuttle launches and quickly skimmed through why the earth began to rotate in the first place.

But in the 21st century, there are many experiments  (Coriolis effect,  Foucault’s pendulum ,etc, etc) that you can do to convince yourself that the Earth is indeed rotating.


                                             ISS Live Stream

But back in the days of Galileo it was still debatable whether Earth was rotating or not. 

Why does a ball thrown straight up in the air fall to the same place on the ground ?


of the profound Aristotelian arguments against the rotation of the
Earth was that if the Earth were rotating, a thrown ball/arrow would not
land in the same place that it was thrown.


This was believed so
because, by the time the projectile traverses its path the Earth would
have moved by a certain distance. Hence, the ball would never land at
the same place it was thrown.


Take a second to think about this. Can you come up with an argument against this rationale ?


is an excellent argument given by Galileo in favor of the rotation of
the Earth and why things would still fall to the same place even if the
Earth were rotating:

In replying to this, those who make the earth movable answer that the canon and the ball which are on the earth share its motion or rather that all of them together have the same motion naturally.

Therefore the ball does not start from rest at all but to its motion about the center joins one of projection upward which neither removes not impedes the former.

will see the same thing by making the experiment on a ship with a ball
thrown perpendicularly upward from a catapult. It will return to the
same place whether the ship is moving or standing still

The profundity of this argument is that, the very same principle that ‘ball does not start from rest at all  but with velocity of the earth’ is used by space shuttles to reach orbital velocity with lesser fuel consumption.


But despite Galileo’s argument, it was still believed for a long time that it were the heavens that moved and not the earth.

God hath established the world which shall not be moved in spite of
contrary reasons because they are clearly not conclusive persuasions.



The Tale of Earth and it’s sister Theia.

Have you found it weird that the earth’s axis is tilted by 23.5 degrees and wondered what led to this? Wait, Where did the moon come from?

Well, Here’s what scientists have theorized.

The Giant Impact Hypothesis

Theia, a mars sized planet collided ( it glanced and thankfully did not collide head on, else it would have destroyed earth ) with the Earth around 4.553 billion years ago.


Theia’s debris gathered together around Earth to form what we now call- The Moon.


The collision between the early earth and Theia was so immense that
it tilted the axis of rotation of the early earth by 23.5 degrees.

it remains tilted so that way even today!!


Why do they believe in this hypothesis ?

Scientists have a very good reason to believe in the Giant Impact Hypothesis:

  • Earth’s spin and the Moon’s orbit have similar orientations.
  • Moon samples indicate that the Moon once had a molten surface.
  • The Moon has a relatively small iron core.
  • The Moon has a lower density than Earth.
  • Evidence exists of similar collisions in other star systems (that result in debris disks).
  • Giant collisions are consistent with the leading theories of the formation of the solar system.
  • The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin

Have a good day !

PC: sarice,

Now not only do we now have an understanding of why the earth is spinning but also why the axis of rotation is inclined by 23.5 degrees.

Stay tuned for more..

Why did the Earth start spinning?

We have been discussing about the rotation of the Earth in the past couple of posts but lets take a second to understand why the earth is rotating in the first place.

Here is a quick summary:

Our Solar System formed about 4.6 billion years ago when a huge cloud
of gas and dust started to collapse under its own gravity.


As the cloud collapsed, it started to spin. Some of the material
within this cloud gathered into swirling eddies and eventually formed
into planets. As the planets formed, they kept this spinning motion.


As material gathered in more closely to form a planet, like Earth, the
material spun faster. This is similar to what you see when skaters pull in their arms and spin

And as a result since there are no forces are acting on Earth to slow it down, it continues to spin to this day. **

** not entirely true as we shall explore soon

Previous posts:

The shape of the Earth is an oblate spheroid: A visual demo

How does earth’s rotation affect space shuttle launches ?

Shape of Earth is an oblate spheroid

As you can see from this dramatic demonstration, the consequence of imparting a rotational velocity to an object is the flattening of object at the poles.

The rotational velocity of Earth is a staggering 1,674.4 km/h 
and as a result  bulges at the center by a tiny fraction, as is evident from the following image:


OKay.. THis might seem like a stupid question.. But why doesn’t the earth spiral into the sun ? Like why is orbiting it ?


In a recent post we demonstrated using the Newton’s cannon thought experiment that:

ISS and other satellites are always falling, but they are falling in Orbit

And this applies to planetary bodies too: Earth is always trying to fall into the sun, but it keeps falling in an orbit instead

But where did the initial velocity for the earth come from ?

Well, to answer that question one needs to go back 4.6 Billion years ago and look at the formation of the solar system.


And if you like numerical simulation and are wondering how could a spinning cloud of gas eventually form planets, take a look at the following video and its source paper.

Thanks for asking!

Try bringing two of your fingers closer in the back drop of a light source and you would observe this:


Long before your fingers actually touch, the edges magically seem to touch each other. How is this even possible?


Transit of Venus

When scientists were observing the transit of Venus from Earth i.e when the planet Venus passes directly between the Sun and Earth,they faced a similar problem. 

At the moment when Venus should
nearly touch the edge of the sun, the circular planet began to elongate.


                                                PC: NASA

And they noticed the same phenomenon for Mercury as well (which has no atmosphere).

What is causing this optical phenomenon?

The physics behind this beautifully bizarre optical phenomenon will be revealed tomorrow on FYP!.

But since this is something that you can all try at home, we strongly encourage you to play around with this and get a feel for it. It requires only your hands and a source of light.

Once you do, try to hypothesize  a solution for this behavior.

Have fun!

34 years ago, on this day NASA Astronauts performed the first untethered spacewalk. And yesterday Spacex just put a Tesla Roadster in an orbit to Mars. This is truly an exciting time to be alive.

** Just to be really clear, this is not the first time we are launching “automobiles” to space.


                                        Lunar Roving Vehicle