By Bill Pellerin
Houston Astronomical Society
Early observers of the night sky thought that Earth was at the center of the universe, but now we know better. There is no center of the universe. No matter, in many ways we continue to act as if we’re at the center of the universe. Many of the measures that we talk about as amateur (and professional) astronomers are based on circumstances that are unique to us, Earth, and our solar system.
What happens when humans have populated other planets in other solar systems? What will it mean to talk about a ‘year’? What if we want to communicate our understanding of the universe with intelligent beings on a planet outside our solar system? Ignoring all other barriers to communication, what would it mean to say that a planet is 3 AU from its ‘Sun’?
There are plenty of examples of astronomical measures for which the reference is to things that are almost certainly unique to our solar system. Here are a few examples:
The Astronomical Unit (AU) – The AU is a measure of the average distance of from Earth to the Sun (92,955,807.273 +/- .002 miles). The error of .002 mile equals about 10 feet. Like many of the reference measures we use, the value for the average distance from Earth to the Sun may change. Why? There are numerous pulls and tugs on Earth in its orbit (the Moon, other planets) and over a long period of time Earth’s orbit could get smaller or larger. In any case, the value of the AU would change as Earth’s orbit changes.
In any case the AU would not be meaningful outside our solar system.
A Year -- How many times have we pointed at Vega and said that it is 25 light years away. We all know that this means that the light from the star took 25 (Earth) years to reach our eyes. The speed of light is well known and it is defined by us in miles (or meters, if you prefer) per second. Albert Einstein showed us that the speed of light is the same everywhere. But using years as part of the definition relates the distance to Vega in terms that include the time for Earth to make one revolution around the Sun.
Is there a more universal measure of time? Yes, there is. The standard reference for time involves counting the number of cycles produced by a cesium clock that runs at a frequency of 9.192631770 GHz. That is, it produces 9.19… billion cycles per second, so the second is based on counting those transitions and when the requisite 9.19… billion cycles have happened, we call that one second. Never mind the number of cycles that equals one second. If we wanted to represent a ‘year’ as the number of cycles generated by a cesium clock, we’d have to count 2.9097x1016 (29,097,000,000,000,000) cycles to get there. Ok, the basic interval would be the same anywhere in the universe and is not dependent on Earth’s orbit. We’d need to invent a new word for a measurement of a long period of time to replace the word ‘year’.
Right Ascension / Declination (RA/Dec) -- This is a big one. Earthlings have projected the longitude and latitude lines from Earth onto the sky and (to make things easy?) have changed the measures and names to RA (right ascension) and Dec (declination) respectively. Those of us who observe the sky are very familiar with these measures. The RA is measured in hours, minutes, and seconds with the 0 hour line defining the east-west position of the Sun on the sky at the vernal equinox, the first moment of spring and hours increasing to the east – there are 24 total hours of RA. Dec goes from 0 degrees (at the projection of Earth’s equator) to 90 degrees at the north pole and -90 degrees at the south pole.
This coordinate system being a projection of Earth’s coordinate system creates a problem. Earth’s axis precesses over time; Earth is like a top that is slowing down with the axis of the top is pointing to different locations on the sky over time. One complete precession cycle takes 26,000 years, and the complication this creates for celestial coordinates is that Earth’s coordinate lines are no longer in sync with the sky coordinate lines. In the past, we’ve simply accepted this problem and re-assigned RA and Dec coordinates to objects in the sky. Most of us now use year 2000 stellar coordinates, and it’s likely that new coordinates will have to be applied perhaps for the year 2050. There will be equations to convert from one coordinate year to another, and the positions imbedded with previously measured objects will have to be corrected as new measurements are taken. It remains a bit of a nuisance to be obliged to reconcile observations from different epochs because of this problem.
Other coordinate systems exist – galactic coordinates are available, and they’re based on the major axis of the Milky Way galaxy. Better, because this system isn’t Earth coordinate based, but this system continues to use as a reference something rather local to us – the Milky Way.
Solar mass – Read any book on stellar physics and you’ll see a lot of reference to stars that are compared to the Sun. The literature is replete with references to one solar mass stars, 10 solar mass stars, and so on. Stars more massive than 8 solar masses (or thereabouts) are referred to as high-mass stars; those less massive are called low-mass stars. A civilization orbiting another star would not be able to relate to the notion of solar masses, since their central star would almost certainly be of a different mass.
Parallax and Parsec – for stars that are relatively nearby, we can determine the distance to the star by measuring the parallax that we see when we take two observations 6 months apart. The idea is to measure the apparent shift in a star’s position over that time and use trigonometry to determine the distance to the star. The best parallax data is from the Hiparcos satellite (the name means High precision parallax collecting satellite). All well and good.
The Parsec is based on the parallax measurement – a parsec is the distance a star has to be from us to produce 1 arc second of apparent position shift in measurements taken 6 months apart. One parsec is about 3.26 light years and is this value only because of the size of Earth’s orbit.
As we look at the night sky and as we talk about objects in the sky we have to keep in mind that we’re observing from one small planet in one solar system in one galaxy in a very large universe. If we ever have the opportunity to move to another planet in another star system or communicate with another planet we will have to make our descriptions of the cosmos more universal.