Frequently Asked Questions

Accuracy Questions

I'm using Voyager or SkyGazer to predict the position of an asteroid, comet, or spacecraft, but the position Voyager gives me is significantly different from what I've found in another source (like a magazine, or an on-line ephemeris). What's the problem?

This is almost certainly the result of using outdated orbital elements for the asteroid, comet, or spacecraft in question. Voyager 4.5 and SkyGazer 4.5 lets you automatically download and import the latest comet/asteroid/satellite orbit files from the Minor Planet Center and other sources; use the Updates command in the Voyager 4.5 or SkyGazer 4.5 menu (Mac OS X) or Help menu (Windows). Please note that your software must be registered in order to download data updates; unregistered versions have a fixed set of orbits that cannot be updated.

Because of the gravitational perturbations of the other planets, the orbit of any object in the solar system is always changing. Therefore, a single set of orbital elements will gradually become less accurate over time. However, most asteroid and comet orbits change slowly enough that predictions based on a single orbit should be good to about an arcminute for several years, which ought to be sufficient for locating them in the sky.

Artificial satellite orbits tend to decay rather quickly, so it is important to use up-to-date orbits. As a rule of thumb, satellite orbital elements should be considered "out of date" and not trustworthy for predictions after 30 days.


The time of sunrise/sunset (or moonrise/moonset) predicted by Voyager or SkyGazer differs from what I see printed in my local newspaper, or from what I observed with my eyeballs. Why? Who's right? Who's wrong?

First, make sure that your longitude, latitude, and time zone are entered correctly. Elevation above sea level doesn't make too much difference, but it helps to have that correct as well.

If your sunrise/sunset times are off by exactly one hour, the problem is almost certainly that your time zone is entered incorrectly. Another possibility is that Voyager is assuming daylight savings time is in effect when it really isn't, or vice-versa. For most of the USA, Canada, Mexico, Western Europe, Australia, Chile, New Zealand, and other parts of the world which push their clocks forward by one hour during summer months, Voyager can automatically predict when daylight savings time is in effect, and when it isn't.

However, governments sometimes "change the rules" with regard to daylight savings time, and although we've tried to keep our software current, we might have missed something. If you're sure that DLST is not in effect - for example, if your location does not use DLST - then turn off the "Automatic Daylight Savings Time" option. If you are absolutely certain that daylight savings time is in effect, turn off the "Automatic Daylight Savings Time" option, and add one hour to your time zone.

Beyond that, it's important to note that your observed sunrise/sunset times can differ from their predicted times by up to several minutes. Voyager (and your local newspaper, and for that matter the US Naval Observatory) predict rise/set times assuming a perfectly flat, sea-level horizon, and typical atmospheric refraction. Your horizon may not be perfectly flat; there may be clouds, trees, or other obstructions; the atmospheric temperature and/or pressure may be unusually high or low, leading to unusual refractive conditions, etc. All of these things can add up to several minutes.

Finally, note that sunrise/sunset (or moonrise/moonset) times are officially defined as the instants that the upper limb of the sun/moon appears on the (mathematically ideal) horizon, not when the center of the solar or lunar disk is on the horizon.


How accurately can Voyager or SkyGazer predict the positions of the Sun/Moon/planets many thousands of years in the past? Or in the future?

For times up to about 10,000 years from the present, Voyager 4.5 and SkyGazer 4.5 make predictions based on JPL's high-precision planetary ephemeris, and their predictions will agree with the positions listed in the Astronomical Almanac to a small fraction of an arcsecond.

While you can use Voyager 4.5 to predict the positions of the planets and moons over a million-year timespan from 498,000 B.C. to 502,000 A.D, its predictions should only be considered "trustworthy" within about 10,000 years of the present. The fundamental problem is that the motions of the planets are chaotic -- that is, essentially unpredictable -- in the long term. We do not know the current positions, masses, and velocities of all planets and moons in the solar system to calculate their positions with arcsecond precision more than a few thousand years from the present. For instance, the position of Saturn's moons is affected by the gravity of Saturn's rings, which depends on the mass of Saturn's rings -- and that is an all-but-unknown quantity. Similarly, while we do know that 500,000 years ago, the Earth was in an orbit roughly the same size and shape as it is today, we simply can't predict which side of the Sun it was on.

For predicting the local circumstances of celestial events over great timespans - for example, determining moonrise/moonset times millennia in the future, or determining if a particular solar eclipse was visible from a particular location many thousands of years ago - the biggest "unknown variable" is actually the rotation of the Earth. Due to its gravitational interaction with the Moon, the Earth's rotation is gradually slowing - but it is slowing irregularly. We periodically add or subtract "leap seconds" to/from our civil time scale, to keep it in sync with where the Earth is actually pointing. But over time, these accumulated time corrections add up. In 4000 B.C. the time correction increases to about 2 days, and in much earlier periods the local time of an astronomical event is only an educated guess. The values for periods before 500 B.C. are very approximate, and they have uncertainties of 10% or more.

Note that this problem affects the local circumstances of an astronomical event (such as an object's altitude/azimuth relative to the local horizon; or its rise/set times). It also affects the calculated equatorial coordinates of the Moon and planets when using Universal Coordinated Time (UTC), which is based on the rotation of the Earth, and is the basis for the world's civil time scale. To take the irregularities in the Earth's rotation out of the equation, turn off the "Automatic Delta T" option (in the Chart menu > Set Date and Time... command > Set Time dialog > Other tab. Then Voyager will be using Terrestrial Dynamic Time (TDT) - this is the perfectly regular time scale that JPL uses to compute planetary positions, and against which the ephemerides in the Astronomical Almanac are tabulated. As of this writing (late 2008), TDT differs from UTC by about 65 seconds - but that value was much greater in the distant past (as mentioned above), and will change again in the future as leap seconds are added to our clocks.

Please note: the Delta T option is only available in Voyager. SkyGazer always uses UTC so that its results will always be calibrated to civil time.


Why does a planet, moon, or satellite sometimes appear to be "off its orbit"? Why don't the orbital elements presented in the Info Panel match the planet's actual position?

As mentioned above, Voyager and SkyGazer use JPL's high-precision DE408 planetary ephemeris for computing the positions of the Moon and planets. It does not use their orbital elements. However, Voyager needs some set of orbital elements in order to draw a planet's orbit. For that purpose, it uses a set of "mean" or average values for the elements. These mean values match the long-term (secular) variations in the elements, but do not include any of the short-term (periodic) variations. Thus these orbits won't quite match the planets' actual positions. These mean orbital elements are also what the Info Panel displays for planets, in the "Orbit" tab.

This problem also exists for artificial satellites - Voyager uses the full-precision SGP4/SDP4 orbit models to compute satellite positions, but only uses mean elements to display their orbits. For asteroids and comets, however, there is no discrepancy: the orbital elements shown in the Info Panel - and in Voyager's sky chart windows - are used to compute those objects' positions directly.


I've set the time to several thousand years in the past (or future), and now I find that the seasons are "out of sync" with the calendar. For example, around 11,000 BC, the Sun is at the summer solstice around September 21st, not July 21st. What's going on here?

The answer has to do with the calendar. The exact length of the tropical year - that is, the length of time it takes the Sun to make one complete circuit around the ecliptic, from vernal equinox to vernal equinox - is about 365.2422... days. Our modern Gregorian calendar has complicated leap year rules (e.g. leap year every 4th year, but not every 100th, unless every 400th) to make the average length of the calendar year come out very close to the tropical year. However, the Gregorian calendar was only introduced in 1582. Before that, the western world used the Julian calendar, which has much simpler leap year rules (leap years only every 4th year) and thus an average year length of exactly 365.25 days. As a result, the Julian calendar gradually slips out of sync with regard to the seasons over thousands of years. That was the reason the Gregorian Calendar was invented in the first place!

So, the answer is that Voyager is doing the right thing - for dates like 11000 BC, there was no Gregorian calendar; Voyager assumes the date is in the Julian calendar. And the Julian calendar does not match the actual length of the year very well. So according to a strict interpretation of the Julian calendar date, the Summer solstice would indeed have occurred in the month of September. But remember - you're specifying a date in a calendar system more than 8000 years before that calendar was even invented. And the same problem exists in the future, too - our modern Gregorian calendar does not quite match the exact length of the tropical year either, so it, too, will gradually fall out of sync with the seasons.


I don't understand how Voyager or SkyGazer handles precession. (What exactly is precession, anyhow?)

Precession is a very slow "wobble" in the direction of the Earth's rotational axis, which takes about 26,000 years to complete one cycle. As a result of precession, the Earth has had a succession of different "Pole Stars" in the past and future. Polaris, which currently lies above the Earth's north pole, was nearly 4 degrees away from the pole at the time of Copernicus and Columbus. When the Great Pyramid of Gizah was built in Egypt, around 3500 BC, Thuban was about 3-1/2 degrees from the pole. The brilliant star Vega will be about 5 degrees from the pole in 14,000 A.D.

The International Astronomical Union provides a standard formula for computing the effects of precession; this is known as the IAU 1976 expression, and it is what most astronomy programs use. Unfortunately, the IAU 1976 formula is only strictly valid for the years 1600 - 2100 A.D. For time intervals more than a few thousand years from the present, the IAU 1976 formula "blows up" and gives very unrealistic results, such as the Earth being "upside down". For that reason, Voyager 4.5 uses a different set of expressions for precession, provided by W. M. Owen of NASA's Jet Propulsion Laboratory. Dr. Owen's precession formula is based on the same underlying physical model as the IAU 1976 formula, but is valid over a much longer time span. It provides realistic results over a period of 500,000 years from the present, and agrees with the IAU 1976 expression to a precision of a few milliarcseconds for the years 1600 - 2100.

The Earth's axis defines both the equatorial and ecliptic coordinate systems. So, the coordinates of objects in those systems are always changing - not because the objects are moving, but because the coordinate systems are moving. To unambiguously describe an object's position in equatorial or ecliptic coordinates, you also need to specify the epoch (i.e. year) for that coordinate system as well. Coordinates are usually given for "standard epochs" such as 1950, 2000, etc. - but are sometimes also given for the "current epoch" (which is roughly 2008.4 at the time of this writing).

You can choose the precession epoch in which to display equatorial and ecliptic coordinates using the Precession command in the Chart menu. If you select the "use current chart date" option, then the precession epoch is set to the chart's current date and time.

Galactic coordinates are not tied to the motion of the Earth's axis, so precession has no effect on that coordinate system. Just to make things more complicated, for altazimuth (i.e. local horizon) coordinates, precession is always computed for the current chart date, regardless of the epoch you've selected. This ensures that the positions of objects relative to the local horizon are always correct. Using a fixed precession epoch (e.g. 2000.0) when computing altazimuth coordinates hundreds or thousands of years in the past would generate misleading results like the Sun being visible at midnight.


Copyright February, 2011

Carina Software & Instruments, Inc