# Thread: The Big Day: which day of the year is the most time dilated for the distant observer?

1. ## The Big Day: which day of the year is the most time dilated for the distant observer?

Say for someone watching Earth from a half a light year away, which day of the orbit lasts the longest?

The orbit of Earth is slightly elliptical, so would the day when it was closest to the Sun be the most time dilated, in terms of speed dilation, and being deeper in the Sun's gravity well?

2. Established Member
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Originally Posted by Mudskipper
Say for someone watching Earth from a half a light year away, which day of the orbit lasts the longest?

The orbit of Earth is slightly elliptical, so would the day when it was closest to the Sun be the most time dilated, in terms of speed dilation, and being deeper in the Sun's gravity well?
Whats our viewpoint? Are we looking down from the north poles or are we looking from along earths orbital plane?

3. Originally Posted by DaCaptain
Whats our viewpoint? Are we looking down from the north poles or are we looking from along earths orbital plane?
I'm not sure it matters. At half a light years distance, if in fairly flat space, the Earth's revolution should appear to slow down a smidgen(unmeasurable amount)..I think.

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## Wha?

Originally Posted by Mudskipper
Say for someone watching Earth from a half a light year away, which day of the orbit lasts the longest?

The orbit of Earth is slightly elliptical, so would the day when it was closest to the Sun be the most time dilated, in terms of speed dilation, and being deeper in the Sun's gravity well?
Time dilation is a function of relative velocities, not of distance.

5. Any time dilation of a clock on Earth as observed from afar should be maximum at perihelion. That is where it is deepest in the Sun's gravitational well and also moving fastest. If I am not mistaken, both of those effects act in the same direction.

6. Originally Posted by Hornblower
Any time dilation of a clock on Earth as observed from afar should be maximum at perihelion. That is where it is deepest in the Sun's gravitational well and also moving fastest. If I am not mistaken, both of those effects act in the same direction.
Yes, both would be additive to the time dilation, though they oppose each other for orbiting satellites above. I would assume the variation in the Earth-Moon barycenter could also be included, but are these gravity time effects extra tiny compared to the tiny time dilation due to orbital speed?

7. It also depends on how you're measuring the duration of "day" - tropical or sidereal. At perihelion, the planet is moving the fastest in its orbit, which also means it has to turn further to put the sun in the same position in the sky as it was the day before (tropical day). This is the source of the Analemma.

8. IIRC Perihelion occurs in January. I forget the exact day.

9. Originally Posted by Tom Mazanec
IIRC Perihelion occurs in January. I forget the exact day.
It varies from year to year. In the current epoch, either the 2nd, 3rd or 4th of January.

Grant Hutchison

10. Originally Posted by grant hutchison
It varies from year to year. In the current epoch, either the 2nd, 3rd or 4th of January. Grant Hutchison
Last year in Australia perihelion was on 5 Jan, but in 2018 it retreats to 3 Jan. I can get why it would change after a leap day, but not why it retreats without a leap day.

https://www.timeanddate.com/astronom...-solstice.html will vary by your longitude. For me it gives this table
Year Perihelion Distance Aphelion Distance
2017 5 January 2017 1:17 am 147,100,998 km 4 July 2017 6:11 am 152,092,504 km
2018 3 January 2018 4:34 pm 147,097,233 km 7 July 2018 2:46 am 152,095,566 km
2019 3 January 2019 4:19 pm 147,099,760 km 5 July 2019 8:10 am 152,104,285 km
2020 5 January 2020 6:47 pm 147,091,144 km 4 July 2020 9:34 pm 152,095,295 km
2021 3 January 2021 12:50 am 147,093,163 km 6 July 2021 8:27 am 152,100,527 km
* All aphelion/perihelion times are in local Sydney time.
On average perihelion advances by one day every 59 years. It is a primary marker of Milankovic Climate Cycles.

11. Originally Posted by Robert Tulip
Last year in Australia perihelion was on 5 Jan, but in 2018 it retreats to 3 Jan. I can get why it would change after a leap day, but not why it retreats without a leap day.

https://www.timeanddate.com/astronom...-solstice.html will vary by your longitude. For me it gives this table

On average perihelion advances by one day every 59 years. It is a primary marker of Milankovic Climate Cycles.
It'll vary because of the time zone differences, but also by the day of the lunar month. During a full moon, it's closer to the sun, and during a new moon, the earth is farther away from the sun--and there are smaller perturbations from the other planets.

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