The length of time between two successive crossings of the Sun from the celestial meridian is a whereas the length of time it takes Earth to return to the same orientation with respect to the fixed stars is a On Earth, a is four minutes shorter than a.

The answer is  a.first and third quarters 

All these statements are related to drake's equation. it is a probabilistic equation that shows an aproximate number of civilizations in our galaxy that we can communicate with. all the numbers that are inserted into equation are not precisely known so the result varies from 1000 to 100,000,000. the equations was not created to calculate exact number of extraterrestrial civilizations but to increase scientific knowledge how to find them.the equation is: where: n=  represents the number of civilizations in our galaxy that we could in principle communicate with today.  the average rate of  star formation  in  our galaxy  the fraction of those stars that have  planets  represents the number of planets in the galaxy that are in principle capable of having surface liquid water and life.  essentially represents the probability that life eventually arises on a habitable planet.  represents the number of life-bearing planets in the galaxy.  epresents the total number of planets on which intelligent beings have evolved and developed a communicating civilization at some time in the galaxy's history.    essentially represents the probability that on a planet with life, some organisms will eventually evolve to the point where they can build radio telescopes.    is a factor that depends on how long civilizations that reach our level of technology are able to survive.