Notebook

What Is Time ?

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from IPython.display import HTML

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HTML('<iframe src=http://raphaeljs.com/polar-clock.html width=630 height=630></iframe>')

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HTML('<iframe src=http://randomibis.com/coolclock/justclock.html width=340 height=340></iframe>')

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HTML('<iframe src=http://www.uize.com/examples/digital-clock.html width=850 height=600></iframe>')

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Time always ticks and never stops¶

In computer world, time starts at 12:00am, January 1, 1970 (epoch). Every time interval is a tick or second.

In python world, there are 3 basic modules to track time:

Question: how many ticks are there since epoch?

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import time
nticks = time.time()
print "It is now %f ticks (seconds) past epoch" % nticks

It is now 1353713584.312000 ticks (seconds) past epoch

Time is structured as a tuple of 9 pieces¶

Index	Attribute	Meaning	Value	Range
0	tm_year	4-digit year	2012	1 - 9999
1	tm_mon	montd	11	1 - 12
2	tm_mday	day of montd	23	1 - 31
3	tm_hour	24 hour	18	0 - 23
4	tm_min	minute	52	0 - 59
5	tm_sec	second	6	0 - 61 (60 or 61 are leap seconds)
6	tm_wday	day of week	4	0 - 6 (0 for Monday)
7	tm_yday	day of year	328	1 - 366 (Julian day)
8	tm_isdst	daylight savings	0	-1, 0, 1 (-1 means library determines DST)

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print "Year (Min, Max): (%d, %d)" % (datetime.MINYEAR, datetime.MAXYEAR)

Year (Min, Max): (1, 9999)

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localtime = time.localtime(time.time())
print "local time is:\n\t", localtime

local time is:
	time.struct_time(tm_year=2012, tm_mon=11, tm_mday=23, tm_hour=19, tm_min=3, tm_sec=19, tm_wday=4, tm_yday=328, tm_isdst=0)

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print "local time is:\n\t", time.asctime(localtime)

local time is:
	Fri Nov 23 19:03:19 2012

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import datetime

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# what timezone are you in
print time.tzname

('Eastern Standard Time', 'Eastern Daylight Time')

Time units¶

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The Equation of Time¶

The equation of time (EOT) is a formula used in the process of converting between solar time and clock time to compensate for the earth's elliptical orbit around the sun and its axial tilt. Essentially, the earth does not move perfectly smoothly in a perfectly circular orbit, so the EOT adjusts for that.

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N=np.arange(361)
B=360*(N-81)/365.0 * np.pi/180.0
E=9.87*np.sin(2*B) - 7.53*np.cos(B) - 1.5*np.sin(B)

fig=plt.figure()
ax=fig.add_subplot(111)
ax.grid()
ax.set_xlim(-5,365)
ax.set_ylim(-17,20)
ax.set_xlabel('Day of Year')
ax.set_ylabel('Time Adjustment (min)')
ax.set_title('Equation of Time')
ax.plot(N,E,'ro')
plt.show()

The EOT can be approximated by the following formula:

$$ E = 9.87 sin(2B) - 7.53 cos(B) - 1.5 sin(B) $$

where $ B = \frac{360}{365} (N-81) $ in degree and $N$ is the day of year.

Show me a calendar¶

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import calendar
# set Sunday as 1st column
calendar.setfirstweekday(6)
# show calendar for a month
cal = calendar.month(2012,11)
print cal

   November 2012
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30

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# show calendar for a year
cal2 = calendar.calendar(2012)
print cal2

                                  2012

      January                   February                   March
Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7                1  2  3  4                   1  2  3
 8  9 10 11 12 13 14       5  6  7  8  9 10 11       4  5  6  7  8  9 10
15 16 17 18 19 20 21      12 13 14 15 16 17 18      11 12 13 14 15 16 17
22 23 24 25 26 27 28      19 20 21 22 23 24 25      18 19 20 21 22 23 24
29 30 31                  26 27 28 29               25 26 27 28 29 30 31

       April                      May                       June
Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7             1  2  3  4  5                      1  2
 8  9 10 11 12 13 14       6  7  8  9 10 11 12       3  4  5  6  7  8  9
15 16 17 18 19 20 21      13 14 15 16 17 18 19      10 11 12 13 14 15 16
22 23 24 25 26 27 28      20 21 22 23 24 25 26      17 18 19 20 21 22 23
29 30                     27 28 29 30 31            24 25 26 27 28 29 30

        July                     August                  September
Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7                1  2  3  4                         1
 8  9 10 11 12 13 14       5  6  7  8  9 10 11       2  3  4  5  6  7  8
15 16 17 18 19 20 21      12 13 14 15 16 17 18       9 10 11 12 13 14 15
22 23 24 25 26 27 28      19 20 21 22 23 24 25      16 17 18 19 20 21 22
29 30 31                  26 27 28 29 30 31         23 24 25 26 27 28 29
                                                    30

      October                   November                  December
Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa      Su Mo Tu We Th Fr Sa
    1  2  3  4  5  6                   1  2  3                         1
 7  8  9 10 11 12 13       4  5  6  7  8  9 10       2  3  4  5  6  7  8
14 15 16 17 18 19 20      11 12 13 14 15 16 17       9 10 11 12 13 14 15
21 22 23 24 25 26 27      18 19 20 21 22 23 24      16 17 18 19 20 21 22
28 29 30 31               25 26 27 28 29 30         23 24 25 26 27 28 29
                                                    30 31

History of time-keeping¶

Story of Time by a modern string physicist¶

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from IPython.display import YouTubeVideo
YouTubeVideo('MTx6ha6fRwo')

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watch the rest: Time - a BBC documentary

References¶

Any feedback is welcome ¶

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