Why does Sun/Moon appear bigger at the horizon?

You might have seen some pictures like below one depicting huge moon/sun at the horizon. Ever wondered why does it appear so big?

In reality, their size remains the same. The effect is caused by an optical illusion popularly known as Ebbinghaus illusion. 

The illusion is better explained with the below figure. 

Those two circles in the middle are of the same size, but the second one seems bigger, because the things around it are smaller.

The moon/sun would appear bigger near the horizon because there are things way off in the distance that appear very small next to the moon, creating the Ebbinghaus illusion.

Why cats always land on their feet?

If you had ever owned a cat then you would have definitely tried dropping it from a height and somehow it would always managed to land on its feet. Ever wondered why?

The cat righting reflex is a cat's innate ability to orient itself as it falls in order to land on its feet. Cats can pull it off so easily because they have very flexible backbone and no functional clavicle.

They are able to accomplish this with these key steps:

1. Bend in the middle so that the front half of their body rotates about a different axis from the rear half.
2. Tuck their front legs in to reduce the moment of inertia of the front half of their body and extend their rear legs to increase the moment of inertia of the rear half of their body so that they can rotate their front further (as much as 90°) while the rear half rotates in the opposite direction less (as little as 10°).
3. Extend their front legs and tuck their rear legs so that they can rotate their rear half further while their front half rotates in the opposite direction less.

And the minimum height at which they can execute this perfectly is 30 cm.

Don't own a cat? Cats are cool.. Adopt one. And you can experiment on it.  

Kidding. 

Source: Wikipedia.

Why does lightning form in a zig-zagged line?

Ever wondered why lightning never forms in a straight line? The reason is simple.

This is similar to the flow of river.  The water of a waterfall goes in a straight line because there is no obstruction to it. But the water of a river on a slope follows a zig-zag line called meandering.  In lightning that phenomenon is called tortuosity.

Lightning is just an electricity. A huge chunk of charged electrons trying to find the path of least resistance to reach the ground. The electric arc forms where the air can be easily ionized and generally in the direction of shortest path between the electric field and the ground.

To know more about how does the lightning know where to go  click on this thread on Stackexchange.
http://physics.stackexchange.com/questions/55340/how-does-lightning-know-where-to-go

How do ants get back home?

Ants are fascinating species. They are disciplined, social and amazingly strong. You saw that in Antman movie, didn't you?

An ant colony starts out looking for food by sending scouts ants or the search party. These ants disperse in random directions searching for food. But when they find the food how do they return home?  Visuals helps sometimes but ants are very small creatures and have a smaller field of view than humans.


The better method they approach is by using a trail of pheromone which is chemical scent. When the scouts leave home searching for food they leave a trail of pheromone along the way.  When they find the food, they follow this scented marks back home. If any other scout ant which is in pursuit of  food comes across any pheromone trail it joins the trail and follows the path. If the path reaches the nest then it follows the path in the opposite direction leaving the pheromone on the way and thus making the trail stronger.

Next time when you see a scout ant on the floor, just wipe the floor or mark it with some other stronger scent like perfume and have fun watching the ant not knowing how to get back home.

Kidding, don't do that. Don't be a sadist.

How to spot ISS?

ISS is a great example of international collaboration. I love watching night sky and whenever I do, I keep an on ISS.

ISS orbits around Earth at a speed of roughly 17,150 miles per hour. That means, it orbits around Earth every 92 minutes. Yes, Astronauts onboard see a sunrise every 92 minutes.

Now, if you are in the red part of the below image you can watch the ISS at night. India being center of that part has one of the best views.

You can spot the ISS using the very cool NASA webpage.

http://spotthestation.nasa.gov/

In the below example in 2 minutes window you can watch ISS appear at 10degrees about North of North-East and  it will continue till 12 degrees above East of Nort East.

There are many apps which provide the current location of ISS.
AppStore - https://itunes.apple.com/en/app/iss-spotter/id523486350?mt=8
PlayStore - https://play.google.com/store/apps/details?id=com.runar.issdetector&hl=en


For better understanding of why ISS is visible for short duration, checkout Rober Frost's answer on Quora.

https://www.quora.com/How-can-I-see-the-ISS-from-my-city/answer/Robert-Frost-1


So, what are you waiting for? Get out there and spot the ISS.

Who will survive nuclear apocalypse?

You might have guessed it right. Cockroaches.

Cockroaches are remarkably tolerant to radiation. A rem of 800 (A measure of radiation exposure) is deadly for humans. Cockroaches can survive upto exposure of 70,000 and above. That's a lot of radiation.

Why?

Slower cell cycles:

Cells are most sensitive to radiation when they are dividing. Some of the human cells constantly divide, thus providing more room for the radiation attack. But because of the simple bodies and slower cell cycles of cockroaches which moult about once a week at the least, cockroaches pose greater opposition to the radiation.

Now that does not mean heat cant kill it. In case of a nuclear blast cockroach has to find a distant safer place far enough from the blast radius to survive. 

Why do some jets leave white trail in the sky?

No, those trail are not the smoke from the plane engine. They are called Contrails, shortened for condensation trail. The science behind it is same as on certain days in winter when you exhale you can see your breath.

Jet fuel is actually made of carbon and hydrogen. When it burns with oxygen, CO2 is formed along with H2O. When this water mixes with the wet cold air, it condenses and ice crystals are formed. How long will these contrails last depends on how wet and cold is the surrounding air. When the conditions are pleasant, they spread across to look like Cirrus clouds.

Now, why do only few planes leave contrails?
1. Altitude - Contrails usually form at altitude above 20000 feet where the air is at -40 to -60 degree Celcius.
2. Engines - Some engines leave contrails and some don't.
3. Dry air - Contrails are not formed when the air is dry.