accumulated discrepancy between measurements using julian years and solar years

blatherskite · 27-09-2012 01:44PM #1

When astronomers talk about events in the distant past such as the birth of the universe or the solar system, they use the Julian year of 365.25 days (31,557,600 S.I seconds). However, the average length of a year in the Gregorian calender which we use is 365.2425 days (31,556,952 S.I seconds); for every 1,000,000,000 julian years, there are 1,000,020,534.3 solar years. Is this difference considered negligible by the astronomers? It means that if the universe is 13.7 billion years old, the universe is about 281,320 solar years older than what measurements using the Julian year state.

edit: sorry for editing a few times, got the sums wrong!

27-09-2012 02:01PM

It's pretty negligible considering estimates of the age of the universe or solar system are just that: estimates.

The universe is 13.75 +/- .11 billion years. There's a + or - allowance of 110,000,000 years with the current estimate.

The difference between using Julian and Gregorian calenders is probably more pronounced when it comes to estimating the age of the Solar System, considering it can be dated more accurately.

Apart from the differences between using a Gregorian or Julian calender, there's a lot more that goes into estimating the length of a day or a year. They're surprisingly complicated, yet deceptively simple, concepts.

https://www.youtube.com/watch?v=dCyTNpcDTUM

https://www.youtube.com/watch?v=IhqzW97_47w

blatherskite · 27-09-2012 02:19PM

^ that 'year on earth' video is brilliant

blatherskite · 27-09-2012 10:15PM

Another thing I was thinking of (don't want to start another thread):

as the sun burns itself out and loses mass, does the earth's orbit change as its gravitational attraction to the sun lessens ie. does the earth drift further away? So in a billion years, will a year be longer than it is now? And was the length of a year shorter when the sun had a greater mass? If this is the case, are there any rough number estimates for the rate at which the actual length of a year (not the length of a day) is changing?

blatherskite · 27-09-2012 10:22PM

Just found this article, which says the earth *is* moving away from the sun, at a rate of about 15cm a year

http://www.newscientist.com/article/dn17228-why-is-the-earth-moving-away-from-the-sun.html

the sun and Earth are literally pushing each other away due to their tidal interaction.

It's the same process that's gradually driving the moon's orbit outward: Tides raised by the moon in our oceans are gradually transferring Earth's rotational energy to lunar motion. As a consequence, each year the moon's orbit expands by about 4 cm and Earth's rotation slows by 0.000017 second.

Likewise, Miura's team assumes that our planet's mass is raising a tiny but sustained tidal bulge in the sun. They calculate that, thanks to Earth, the sun's rotation rate is slowing by 3 milliseconds per century (0.00003 second per year). According to their explanation, the distance between the Earth and sun is growing because the sun is losing its angular momentum.

bnt · 27-09-2012 10:24PM

This is why leap seconds are applied occasionally. It's not a regular thing e.g. have a look at the chart on that page. Leap seconds are applied every 1-2 years, but there were none needed from 1999-2004 and 2009-11.

blatherskite · 27-09-2012 10:25PM

bnt wrote: »

This is why leap seconds are applied occasionally. It's not a regular thing e.g. have a look at the chart on that page. Leap seconds are applied every 1-2 years, but there were none needed from 1999-2004 and 2009-11.

I may be corrected, but I thought leap seconds made up for the fact that the length of a day is growing, rather than the length of a year? apparently the average length of a day will be a second longer in 50,000 years and adding leap seconds every so often just resets the effect that this has on civil calender

bnt · 28-09-2012 08:30PM

Well, I thought the effect would be on both - since the Solar Year is defined in terms of days anyway. Never mind.

krd · 30-09-2012 02:52PM

blatherskite wrote: »

I may be corrected, but I thought leap seconds made up for the fact that the length of a day is growing, rather than the length of a year? apparently the average length of a day will be a second longer in 50,000 years and adding leap seconds every so often just resets the effect that this has on civil calender

Yes, because the sun is slowing down the rotation of the earth.

Like the earth slowed the rotation of the moon to a stand still.

Days on earth will become slower and slower, until it takes them years to pass - and then eventually the rotation of the earth will grind to a halt, and one half of the planet will be in permanent day, and the other in permanent night.

accumulated discrepancy between measurements using julian years and solar years

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