# What can change the duration of day-night(i.e, time) on earth ? i have heard that constructing tall buildings can slightly change the time...but how will that change? does earth axis changes or something related to this? will the centre of gravity affected? pls help me :)

Grade:10

## 3 Answers

Ashwin Muralidharan IIT Madras
290 Points
11 years ago

Hi Amin,

No matter what, the duration of a single day (ie morning+night) will be a constant even if tall buildings are constructed.

That is because the time taken for earth the complete one revolution will still be only 24hrs.

But then, the construction of tall buildings will affect the length of morning (decreasing it) and night (increasing it). But remember the sum would be a constant. ie a day is still only 24 hrs.

The duration of morning will decrease because the rays of the sun will reach earth late (because of the tall  building) and the sun set would also be earlier. Thus decreasing the duration of the morning (ie the presence of sunlight).

Hope it helps.

Wish you all the best.

Regards,

Ashwin (IIT Madras).

Swapnil Saxena
102 Points
11 years ago

This fact can be can be best understood on the basis of law of conservation of angular momenta.

When we construct a tall building we tends increase the bulk or simply mass of the earth at a particular point which is otherwise distributed evenly on its surface. As such now there is a slight increase in the moment of inertia of the system and consequently angular velocity decreases. When angular velocity decreases the time needed by the earth to complete one rotation will increase. However for the increse to be observable, the height of the buliding should be very large and building should be very bulky.

It can also be explained quite mathematically.

Assume thet the initial earth is a uniform sphere(approx.) with mass M. Its moment of inertia is 2/5MR^2

Now we constructed a bulitding of mass m using construction materal which is dispersed on the earth.

Now the moment of inertia of earth is 2/5(M-m)R^2 (R will not chage significantly) . Also the moment of inertia of the building is m(R+H/2)^2. where H is the height of the buliding

So now the anglar momenta of the system is  2/5(M-m)R^2 + m(R+h/2)^2 that would definitely > 2/5MR^2 if the height is significantly large. Then Acc to the law of angular momenta :- The final angular velocity should be less than initial one. Now since the angular velocity has decreased the time should have to increase.

Simply tall builiding tend to increse the moment of inertia of the system due to which its angular velocity  decreases and duration of a day increases.

Sathya
35 Points
10 years ago

when tall building are constructed, soil and minerals taken from mines(deep down the earth) are used.. so moment of inertia increases...so, angular velocity decreases.. hence, duration of day and nite increases.. it is true....