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### Course: Middle school math (India)>Unit 1

Lesson 10: Week 10

# Telling time (unlabeled clock)

Sal tells time on unlabeled analog clocks. Created by Sal Khan.

## Want to join the conversation?

• Is it smarter idea to learn time on a digital watch before an analog watch that tells you the time?
• I agree. It is good to be able to tell time the harder way.
• why do we need A.M. and P.M.?
• We need AM and PM to calculate the days and hours. “AM” stands for the Latin phrase Ante Meridiem —which means “before noon”—and “PM” stands for Post Meridiem : “after noon.”" So if you think of it as AM means morning and PM means night.

Hope this helps!
• When we write time dose the mins come first
• The order is hours, minutes, and seconds. Usually just hours and minutes are used.
• where did 60 seconds in minute, 60 minutes in a hour, and 24 hours in a day come
• Ancient Babylonians. They used number base 60 (Sexagesimal).
• why is time different in different parts of the world shouldn't it just be the same?
It would be a lot less confusing.
• Yes but say if you were in China and someone was in America. For example, AM in America.
For some places at that time, the sun may be setting or just rising.
They also have different time zones so you could have an average like AM -- PM and you could use as much as the daylight as possible.

As mentioned on http://www.timeanddate.com/time/time-zones.html, here's a more in-depth reason:

"Many towns and cities around the world used to set clocks based on observing the sun and the stars. This occurred prior to the late 19th century. Dawn and dusk occur at different times at different places because of the Earth’s rotation. However, time differences between distant locations were barely noticeable because of long travel times and the lack of long-distance communications. The expansion of transport and communications, as well as trade globalization, during the 19th century created a need for a more unified time-keeping system."
• does the second minute hand always move when the second hand counts 60 seconds because it doesnt look like it
(1 vote)
• It sure does. The minute hand will move to the next minute after 60 seconds.
Example:
If the minute hand is on 1, it will be on minute 2 after the second hand counts 60 seconds.
• Is 13 in 24 hour time 1 AM?
(1 vote)
• No it's 1 PM. It works very easily actually when you think about it. Hour 1 in military time (24 hour time I suppose you could say) is 1 AM, the first hour of the day. So 13 hours threw the day would get you to 1 PM. If you can subtract 12 from the number it is in the PM if not it's in the AM. The actual time is minus 12 then just add PM. So again 13-12=xPM x being the number with PM on the end since you can subtract a 12 from 13.
• if it takes Earth 24 hours to spin all the way around (360 degrees) than how many hours, days, weeks, months, or years does it take for Jupiter to completely turn around?
(1 vote)
• Believe it or not, Jupiter actually spins really fast, fast enough that 1 complete rotational period is 10 hours! These speeds also contributes to the constant storms on Jupiter, as well as the Giant Red Spot.