# The Naked Scientists Forum

### Author Topic: What year are we in?  (Read 4290 times)

#### thedoc

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##### What year are we in?
« on: 16/01/2014 11:30:02 »
Connor Raven <> asked the Naked Scientists:

My friend and I are doing some research about what year we are actually in, when taking into account that we measure a day as a solar day whereas really a day should be measured as a sidereal day.

Many Thanks
AK and CR

What do you think?
« Last Edit: 16/01/2014 11:30:02 by _system »

#### CliffordK

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##### Re: What year are we in?
« Reply #1 on: 16/01/2014 14:54:06 »
You might find this discussion about "New Years" interesting.

If you consider the new year as beginning at or around the Winter Solstice, then we probably start the new year about a week late.  However, the point of leap years, leap seconds, and etc is to base the length of day on the rotation of the earth, so the sun is overhead at noon, and on the opposite side of the earth at midnight (not counting daylight savings time).

The year is based on the axial tilt of the planet, so that the solstices and equinoxes occur at the same time each year.

Our current Gregorian Calendar is only a few centuries old, but it is very similar to the Julian Calendar from 45 BC.

The primary problem with the Julian Calendar was an error in the number of leap years.  According to Wikipedia, the two calendars are out of sync by 13 days.  The Russians Orthodox Christmas takes into account this shift in leap years, and considers Christmas to be on January 7, rather than December 25.

Anyway, by now we would be in 2014 in both the Gregorian and Julian Calendars.

Of course, the start date of 2014 years ago is arbitrary, and many other cultures have different years and start dates.  See the list under the Gregorian Calendar Page

#### evan_au

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##### Re: What year are we in?
« Reply #2 on: 16/01/2014 17:27:57 »
The 1 AD reference date for our calendar is nominally based on the birth date of Jesus Christ. (There is no 0 AD in this calendar; immediately before 1 AD is 1 BC.)

Different scholars calculate the dates slightly differently from various historical sources, but the consensus seems to be that Jesus was born around 4BC ± 4 years.

#### evan_au

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##### Re: What year are we in?
« Reply #3 on: 18/01/2014 17:02:38 »
To compare these two definitions of a Day:
• The Solar Day is exactly 24 hours long, and is a measure of the interval between when the Sun is overhead*. This unit is used in most societies.
• The Sidereal Day is about 23 hours, 56 minutes, 4 seconds long and is a measure of the interval between when a star (apart from the Sun) is overhead**. This unit is mostly used by astronomers who want to look at the stars.
Both units involve the rotation rate of the Earth on its axis.

If you add up these 3 minute and 56 second periods, you find that it adds up to 1 day over the course of a year. This merely reflects the fact that over the course of a year, the Earth has completed 1 rotation around the Sun, so there has been 1 additional rotation if you use the stars as your reference point, rather than the Sun.

The Solar year is based on the time for Earth to complete one orbit of the Sun. However, the length of a year (in seconds) is unchanged regardless of whether you measure its duration in Solar Days or Sidereal Days.

So the Year we are in should not be affected by whether you measure the year by Solar Days or Sidereal days.

*Earth's slightly elliptical orbit means that some days are shorter than others, so in practice, the length of a day is calculated as the average over a year..
**Due to wobbles in Earth's spin axis, there is also a  Stellar Day which is 8.4ms longer than a Sidereal day; this will make a difference of 1 day in 26000 years, so it won't affect the year we are in for a very long time.

See: http://en.wikipedia.org/wiki/Sidereal_day
« Last Edit: 18/01/2014 17:12:20 by evan_au »

#### evan_au

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##### Re: What year are we in?
« Reply #4 on: 18/01/2014 17:07:52 »
In the Islamic calendar, the Lunar Year is based on the time for the Moon to complete 12 rotations of the Earth, which is 11 or 12 days longer than a Solar year.

A difference in the length of a year, plus a different reference date means that the Islamic year is different from the Gregorian year.

But the year we are in should not be affected by whether you measure the year by Solar Days or Sidereal days, regardless of whether you use the Gregorian calendar or the Islamic calendar.

See: http://en.wikipedia.org/wiki/Lunar_year

#### alancalverd

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##### Re: What year are we in?
« Reply #5 on: 19/01/2014 00:30:24 »
What year are we in? This one. It doesn't matter what number you give it as long as everyone else uses the same numbering system. Until further notice, the date and time everywhere in the universe is UTC.

#### MrVat7

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##### Re: What year are we in?
« Reply #6 on: 21/01/2014 22:48:00 »
I think 2014 lol :P strange isnt it ?

#### The Naked Scientists Forum

##### Re: What year are we in?
« Reply #6 on: 21/01/2014 22:48:00 »