Leap years because the earth revolves around the sun in less than 365 days
make up for the extra one-fourth day the earth needs to orbit the sun
add an extra day to the end of december, making the month 32 days long
subtract a day from february, making the month only 27 days long

In the Gregorian calendar (the calendar used in most Western countries), the year begins on January 1st and ends on December 31st, lasting 365 days.

However, the Solar year (the time it takes for the Earth to make its orbit around the sun) lasts a little bit longer: 365 days 5 h 48 min 45.10 s (365.242189 days).

This means there is a gap between the Gregorian calendar and the Solar year. So, the solution given to this situation was to correct the calendar every four years by an unaccounted accumulation of approximately 1/4 of a day per year that equals one extra day, as follows:

"Leap year is divisible by 4, unless it is a secular year (last of each century, ending in 00), in which case it must also be divisible by 400."

This extra day is added in February, after day 28th. That is why every four years we have a February 29th in the Gregorian calendars or a Leap year.

d₁ = displacement of tern from arctic to antarctic = 11000 miles

d₂ = displacement of tern from antarctic to arctic =   - 11000 miles

d = total displacement for the journey

total displacement for the journey is given as

d = d₁ + d₂

d = 11000 + (-11000)

d = 0 miles

t₁ = time of travel from arctic to antarctic = 3 months = 3 x 30 days = 90 days

t₂ = time of travel from antarctic to arctic = 3 months = 3 x 30 days = 90 days

t = total time of travel

total time of travel is given as

t = t₁ + t₂

t = 90 + 90

t = 180 days

average velocity is given as

average velocity = total displacement/ total time

v = d/t

v = 0/180

v = 0 miles/day