The Sun, Moon, and stars have helped
people for thousands of years. When you look
into the sky, you will notice that they follow
certain patterns. This happens most of the
time, but not all of the time. Since most
patterns repeat over and over, they have
helped us keep track of time. These patterns
have also helped us make calendars.
Making calendars was a difficult task. Our ancestors had to decide on the length of a year.
Also, they had to decide on the length of a day. As you know a day takes 24
hours. That is the time that Earth takes to rotate once on its axis. A year is longer and
takes 365 days. This length of time is related to the time that it takes for Earth to go
around the Sun. However, this is not exactly true. Earth’s trip takes 365 days – plus a
fraction of another day. This is what complicates our calendar. We have been able to
solve this problem by adding an extra day in the month of February. Most often, you will
see that February has 28 days. Every four years, February has one extra day. During this
year we end up with 29 days in this month. When this happens, the year is called a Leap
Year. It helps us keep our calendar in order.
Deciding on the length of a month has also been a challenge. Some cultures around the
world have used a lunar calendar. A lunar calendar is based on the Moon’s cycle. The
Moon takes 29 and one-half days to complete one cycle. Then things got a lot harder.
Different cultures start their months at a different time of the Moon’s cycle. For example,
the Hebrew and Islamic calendars start their months when a crescent moon is seen in the
sky. The Chinese start their new months at the new moon phase. These differences have
made it harder for cultures to communicate. Trading with one another has also been hard
for this reason.
As time went by, most countries began to use the same calendar. When this happened in
the twentieth century, trade and communication became a lot easier. Some countries
decided to drop thirteen days from their calendar. This is because their old way of
counting did not match up with the new way.
England had problems with the calendar back in the 1500s. The English decided to divide
their calendar into months. Each month had four weeks. By doing this, one week ended
up being longer than seven days. Also, this resulted in thirteen months to a yea
Answer:
Joule - J
Explanation:
As energy is defined via work, the SI unit of energy is the same as the unit of work – the joule (J).
When you have the bottom measurement of the triangle (base) and the height of the triangle in the center, you multiply them together and then divide by 2.
~Do you need more explaining~?
(ANS1)— P4 + 5O2 ---> 2P2O5
(ANS2)— C3H8 + 5O2---> 3CO2 + 4H20
(ANS3)— Ca2Si + 4Cl2 ---> 2CaCl2 + SiCl4
OILRIG:
Oxidation is loss (of electrons)
Reduction is gain (of electrons)
so...
The first one is an oxidation half-equation as the Sn loses electrons;
The second one is a reduction half-equation as the Cl₂ gains electrons
