Does the space shuttle countdown skip the first second after launch (T+0) and if yes why?
You can see this effect in videos like this one:
In comparison with an SpaceX-Falcon-Heavy-Launch for example:
(there is a T+0 second)
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2$\begingroup$ The difference between "ceiling" and "truncate"? Having the clock stop for two seconds looks weird. $\endgroup$ – user20636 Apr 12 '19 at 19:24
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$\begingroup$ Shuttle counted down to 0 and then started counting up, in a straightforward manner. T=0 was an instantaneous point, not something that lasted for a "second" or two. No idea what SpaceX does. $\endgroup$ – Organic Marble Apr 14 '19 at 13:36
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To me this looks like a simple artifact from rounding / truncating as @JCRM commented.
The shuttle countdown seems to show a rounded time, while the SpaceX countdown seems to use a truncated time. The difference gets obvious when we look at the decimals of the seconds:
Time Shuttle Falcon
-2.0 -00:02 -00:02
-1.9 -00:02 -00:01
-1.5 -00:02 -00:01
-1.4 -00:01 -00:01
-1.0 -00:01 -00:01
-0.9 -00:01 -00:00
-0.5 -00:01 -00:00
-0.4 -00:00 -00:00
-0.1 -00:00 -00:00
0 -00:00 00:00
0.1 -00:00 +00:00
0.4 -00:00 +00:00
0.5 +00:01 +00:00
0.9 +00:01 +00:00
1.0 +00:01 +00:01
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SpaceX used trunc(time) or int(time) (thick red line) while NASA used eitherfloor(time) or ceil(time).
I'm sure it can be argued either way from a programmers point of view and also from a stylistic point of view.
import numpy as np
import matplotlib.pyplot as plt
t = np.linspace(-3, 3, 601)
plt.figure()
plt.plot(t, np.floor(t) - 0.1, '-b', linewidth=2)
plt.plot(t, np.trunc(t), '-r', linewidth=4)
plt.plot(t, np.ceil(t) + 0.1, '-g', linewidth=2)
plt.plot(t, t, '-k', linewidth=1)
plt.show()
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$\begingroup$ perhaps you could include an "actual time" line $\endgroup$ – user20636 Apr 14 '19 at 23:54
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$\begingroup$ "SpaceX used int(time) (thick red line) while NASA used floor(time) or ceil(time)." Source? Or are you inferring this? And are you talking just about the time shown on the web videos? $\endgroup$ – Organic Marble Apr 15 '19 at 0:19
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$\begingroup$ @OrganicMarble those are just the names of generic functions. It doesn't matter how they implemented them. $\endgroup$ – uhoh Apr 15 '19 at 13:07
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1$\begingroup$ I've adopted both suggestions. The y axis has units of time, but it's not time. $\endgroup$ – uhoh Apr 15 '19 at 15:48
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$\begingroup$ @uhoh x is "real time", y is "displayed time". You could also add the fourth option, rounded time. $\endgroup$ – asdfex Apr 15 '19 at 17:31
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There is no "skip" of a second. Zero is zero, the time of launch. Attaching a sign to zero is meaningless, and occurs only as a display artifact or an arbitrary choice in rendering the value. For Shuttle, the count-down/count-up time rendering appears to (1) update at every second and (2) simply preserves the minus sign at zero. For SpaceX, it is showing a rounded or truncated second updated at some interval not always on the second, so there is a moment at which it updates where the time is somewhere between T=0 and T=1, rounded/truncated back to 0; thus the rendering removed the minus sign as the moment of launch passed, before the first second had elapsed and been displayed as such.
These countdown displays are for public consumption. Although they may incorporate technical data (e.g. speed and altitude) derived from actual telemetry, there is certain to be some amount of processing once the data is passed from its originating technical domain into the "PR" domain to become the video feed we watch. I would not seek to infer anything of a technical nature from artifacts in the public feed.
