70
votes
Have we attempted to experimentally confirm gravitational time dilation?
You don't need a space probe. Or an aircraft. Or even a car. NIST has measured the predicted general relativity time dilation due to a change in altitude on Earth of one foot!
- 58.5k
52
votes
Have we attempted to experimentally confirm gravitational time dilation?
In addition to specific probes like the one mentioned by called2voyage, the effect is significant enough that it affects everyday operations. For example, the GPS constellation needs regular clock ...
- 1,565
43
votes
Accepted
What does NASA mean when they use the term 'PDT (ERT)'
ERT is Earth Received Time. I.e., when we find out about the event.
source
- 178k
40
votes
Have we attempted to experimentally confirm gravitational time dilation?
Yes, time dilation was experimentally confirmed by Gravity Probe A, launched by NASA on June 18, 1976.
The clock rates of two masers (one on the probe and one on Earth) were compared, and it was ...
- 23.8k
25
votes
Accepted
Why would astronauts need a wristwatch and a separate timer?
(Shuttle-centric answer, but concept likely applies to other spacecraft, especially involving crew who flew on the shuttle)
"Egg timers" were utilized heavily on the shuttle to time multiple ...
- 178k
23
votes
Accepted
New Year's Celebration on the ISS
The official time zone is GMT, or UTC.
As for the number of times they could celebrate, well, that's a bit tricky. The station goes towards the east, which is actually backward in time. There are a ...
- 121k
21
votes
Accepted
How can we transmit a date to another species?
The approach taken by Voyager Golden Record is through use of pulsars. Unlike the insane exponents needed to utilize microscopic physical phenomena or planck time, pulsars operate on ~1 second ...
- 54.9k
21
votes
How do launches avoid leap seconds? Why?
It's not just launches. It's, well, everything. It drives me nuts!
Spacecraft flight software almost always have the capability to execute uplinked commands based on time. But what time scale? The ...
- 73.2k
21
votes
What is the time dilation between Mars and Earth resulting from the mass difference?
From my answer to Parker Solar Probe passing extremely close to the Sun; what relativistic effects will it experience and how large will they be?:
From here (or here if you are ambitious) the lowest ...
- 149k
18
votes
Accepted
Scott Kelly age vs Twin's
Neil DeGrasse Tyson says that Scott Kelly is now 1/100 second younger than he would have been otherwise, which almost certainly isn't enough to alter the birth order of the two; I don't know which of ...
- 167k
18
votes
Accepted
How do launches avoid leap seconds? Why?
Avoiding leap seconds is easy, don't launch at June 30th or December 31st when a leap second is announced, see https://en.wikipedia.org/wiki/Leap_second
It is difficult to test if leap seconds during ...
- 48.8k
17
votes
Accepted
Is GPS time at least "really close" to TAI (International Atomic Time)?
TL;DR: For all practical purposes except milliarcsecond radio astronomy, TAI and GPS time are separated by exactly 19 seconds.
TAI and GPS time both attempt to represent time as ticked by an ideal ...
- 73.2k
16
votes
What decides if a launch has to be done instantaneously or during a window?
In the case of Falcon 9 / Orbcomm OG2 launch from SLC-40, Cape Canaveral Air Force Station, FL that's now scheduled for Dec. 20th at 8:29 p.m. EST (01:29 a.m. UTC), according to Spaceflight Now:
...
- 76.5k
16
votes
How much light is there on the way from Earth to Proxima Centauri?
No, the power collected by solar panels is reduced by the square of the distance from the light source.
At the Earth's distance from the sun, the energy of sunlight is about 1300 watts per square ...
- 167k
16
votes
New Year's Celebration on the ISS
I was interested in the bonus question and decided to do an ad-hoc survey using ISS Tracker in historical mode. I entered the appropriate 2020-01-01 00:00:00 for each apparently relevant time zone (-...
- 377
16
votes
Why would astronauts need a wristwatch and a separate timer?
As mentioned in another answer to your question, astronauts may need to time multiple events. Also using multiple clocks helps to detect if one clock is wrong. In fact Buzz Aldrin famously wears three ...
- 1,977
16
votes
Are there multiple types of UTC time?
I think there's two important ways to think of them:
these are the UTC implementations; when you need to know the time now, precisely, these are the current time references you can use. Actual UTC ...
- 10.1k
15
votes
How does time work on other planets?
Some basics:
Every planet, moon or asteroid have their own rotation period along some specific axis. In very good approximation the period is constant (but not exactly). So every celestial body has ...
- 9,041
14
votes
How to obtain UTC of the epoch time in a satellite TLE (two line element)?
The best guess available about the time scale used in TLE files is from the epic Revisiting Spacetrack Report #3 report from Celestrak, which they have put online here:
https://celestrak.org/...
- 971
13
votes
Accepted
How far from earth have atomic clocks (or ultra-stable oscillators) been placed and monitored?
There was Rb-based Ultra Stable Oscillator in outer space, delivered to Titan (VI moon of Saturn) inside the Huygens, brought there by Cassini. (DWE RUSO in http://nssdc.gsfc.nasa.gov/image/spacecraft/...
- 2,317
13
votes
Accepted
Which Earth time zone most closely matches the ISS astronauts' daily schedule?
The wake/sleep cycle actually does use GMT. It was chosen as a compromise between Moscow time and Houston time. That permits times for meetings between the ISS crew and personnel in either city during ...
- 58.5k
12
votes
Accepted
How are GPS satellite clocks synchronized with each other in the earth's frame of reference?
The satellites' location is very accurately determined using ranging techniques, where a pulse is sent from a ground station to the satellite and the satellite responds very quickly, with a known time ...
- 121k
12
votes
Accepted
Where can the term "Sol" used?
A "sol" is only commonly used for a Martian day, and it seems pretty redundant to use this term for Earth, as it is already covered by the concept 'day'. Also, it may induce ambiguity, as people think ...
12
votes
If the Alcubierre Warp Drive was used to travel to Alpha Centauri, how much time would pass on Earth?
The Alcubierre bubble could get to Alpha Centauri in less than 4 years according to outside observers, not just observers riding inside the bubble--the exact time depends on the exact 'shape' of the ...
- 221
12
votes
Accepted
What kind of time regiment/schedule do ISS astronauts have?
Like a lot of things that should be readily available, it's frustratingly difficult to find this on any NASA web site. Fortunately ESA came to the rescue in the form of a blog post about Alexander ...
- 178k
12
votes
Apollo Missions - Travel time to moon
Escape velocity is the velocity at a given altitude (usually the surface) that is enough to leave the body's sphere of influence with a positive net velocity. But if you leave a body at exactly escape ...
- 4,548
10
votes
How long would it take to travel to Proxima b?
5.8 years
According to this page, distance travelled under constant force acceleration, even up to relativistic speeds, is calculated by:
$s(t) = c(\frac{m_0c}{F})(\sqrt{1 + (\frac{F}{m_0c})^2t^2} - ...
- 1,829
9
votes
How does GPS receiver synchronize time with GPS satellites?
The problem
The GPS receiver's time has to be synchronized with atomic clocks located in GPS satellites. It is kinda chicken or the egg problem. The receiver needs precise time to calculate precise ...
9
votes
Accepted
How to obtain UTC of the epoch time in a satellite TLE (two line element)?
Given: 16031.25992506
The 16 corresponds to 2016. As 1957 was the first year with satellites launched, 57 would be 1957, and in 2057 this might change, as there will be an issue.
The 31 means the ...
- 121k
9
votes
Time, UTC, Julian Date, TLE epoch - how are they related quantitatively?
I often have to go read the Skyfield guide to dates and times to keep everything straight, but, briefly:
A Julian Date is simply a rival way to name moments, that is simpler than our customary date-...
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