most events like the rising and setting of the Sun were used a natural measurement of time until recently.
Solar time, which is based on the motion of the Sun, is not the only way of measuring time, however. One might keep track of the regular appearance of the full Moon. That event occurs once about every 29.5 solar days. The time between appearances of new moons, then, could be used to define a month.
One also can use the position of the stars for measuring time. The system is the same as that used for the Sun, since the Sun itself is a star. All other stars also rise and set on a regular basis.
Although any one of these systems is a satisfactory method for measuring some unit of time, such as a day or a month, the systems may conflict with each other. It is not possible, for example, to fit 365 solar days into 12 or 13 lunar months exactly. This problem creates the need for leap years
Read more: http://www.scienceclarified.com/Ti-Vi/Time.html#ixzz5e1E705sr
I abbreviated most of it but there is a ton more at this link if you still need more.
Answer: -
The approximate number of atoms in a bacterium is 10¹¹
Explanation: -
We are given the mass of a bacterium is 10⁻¹⁵ kg.
We are told that the mass of a hydrogen atom is 10⁻²⁷ kg.
Finally we learn that the average mass of an atom of the bacterium is ten times the mass of a hydrogen atom.
Mass of an atom of bacterium = 10 x mass of hydrogen atom
= 10 x 10⁻²⁷ kg.
= 10⁻²⁶ kg.
Thus the number of atoms in a bacterium = 
= 
= 10¹¹
Isotopes are elements that differ in the number in neutrons while the numner of protons are the same. The answers of the questions are: 1) 18/8 O 2) 22/10 Ne 3) 53/25 Mn 4) 37/24 Cr 5) 106/81 Ti.
By studying seismic waves
Explanation:
Scientists discovered that the earth's outer core is liquid by simply studying seismic waves.
Seismic waves are elastic waves that have been used by geophysicists to study the earth's interior.
- Interior of the earth is inaccessible due to high pressure and temperature conditions therein.
- To study this part of the earth, an indirect method should be used.
- Seismic waves produced during an earthquake due to their permeating nature have been used to study the interior.
- Seismic P-waves can pass through any materials because they are longitudinal waves. They go through solids, liquids and gases.
- S-waves can only pass through solids and not fluids.
- On a seismograph, it was discovered that the S-waves were cut off in a part of the reading.
- The waves re-appeared after a distance.
- Since S-waves cannot pass through liquids, it has been hypothesized that the outer core is liquid.
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