This is the Web Edition of "A Trip Into Space", a Coimbra-based electronic book on space science. Both the texts and the photos are by courtesy of National Aeronautics and Space Administration.

# Notes on the Fact Sheets

```Most values listed in the factsheets are from the following sources:
Astronomical Almanac, 2000
Global Earth Physics, American Geophysical Union, 1995
Astrophysical Quantities, C.W. Allen, 1981

```

### Orbital parameters

```Instantaneous values referenced for Julian Date 2451800.5 (13 September 2000)
[Astromonical Almanac 2000, p. E3]

Semimajor axis (106 km)          Mean distance from the Sun in 10^6 kilometers
Sidereal orbit period (days)     The time it takes the body to make one revolution about
the Sun relative to the fixed stars in days.
Tropical orbit period (days)     The average time for the body to make one revolution
about the Sun from one point in its seasonal orbit to
the equivalent point (e.g. equinox to equinox) in days.
For Earth, this equals exactly 1 year.
Synodic period (days)            The time interval between similar configurations in the
orbit (e.g. opposition) of the body and Earth, in days.
Perihelion (106 km)              The point in a body's orbit closest to the
Sun, in 10^6 kilometers.
Aphelion (106 km)                The point in a body's orbit furthest from
the Sun, in 10^6 kilometers.
Mean orbital velocity (km/s)     The average speed of the body in orbit,
in kilometers/second.
Max. orbital velocity (km/s)     Maximum orbital velocity, at perihelion,
in kilometers/second.
Min. orbital velocity (km/s)     Minimum orbital velocity, at aphelion,
in kilometers/second.
Orbit inclination (deg)          The inclination of the orbit to the ecliptic,
in degrees
Orbit eccentricity               A measure of the circularity of the orbit, equal to
(aphelion - perihelion distance)/(2 x semi-major axis)
For a circular orbit, eccentricity = 0. Dimensionless.
Sidereal rotation period (hrs)   The time for one rotation of the body on its axis
relative to the fixed stars, in hours.  A minus sign
Obliquity to orbit (deg)         The tilt of the body's equator relative to the body's
orbital plane, in degrees.
```

### Mean orbital elements

```250-year least squares fit elements referenced to J2000 (Global Earth Physics, p. 14)

Longitude                          The point in a body's orbit, defined from 0 to 360
degrees.  For bodies orbiting the Sun, the 0 point
of longitude is defined as Earth's vernal equinox.
Longitude of ascending node (deg)  The longitude in a body's orbit at which it crosses
the ecliptic plane with increasing latitude (i.e.
crosses the ecliptic from south to north).
Longitude of perihelion (deg)      The longitude in a body's orbit at which it reaches
the point closest to the Sun.
Mean longitude (deg)               The longitude a body is at in its orbit at 00:00
Universal (Greenwich) Time on January 1, 2000

```

### Atmospheres

```
Surface Pressure:  Atmospheric pressure at the surface, in bars, millibars
(mb = 10^-3 bar), or picobars (10^-12 bar).
Surface Density:  Atmospheric density at the surface in kilograms/meters^3.
Scale height:  The height interval in which the atmospheric pressure changes by a
factor of e = 2.7183
Average temperature:  Mean temperature of the body over the entire surface in
degrees Kelvin.
Diurnal temperature range: Temperature range over an average day in degrees Kelvin.
Wind speeds:  Near surface wind speeds in meters/second
Atmospheric composition:  Relative composition by volume of gasses in the atmosphere.
Mean molecular weight:  Average molecular weight of the atmospheric constituents in
grams/mole
Atmospheric composition (by volume): Relative volume of constituents in the atmosphere,
by percentage or ppm (parts per million).
```

### Related Definitions

```Bar - A measure of pressure or stress.  1 bar = 10^5 Pascal (Pa) = 10^5 kg m^-1 s^-2

Ecliptic - An imaginary plane defined by the Earth's orbit.

Equinox - The point in a body's orbit when the sub-solar point is exactly on the equator.

Gravitational Constant - Relates gravitational force to mass,
= 6.6726 x 10^-11 meters^3 kilograms^-1 seconds^-2

Opposition - An orbital configuration in which two bodies are on exact opposite sides of
the Sun or are on the same side of the Sun forming a line with the Sun
(neglecting inclination)

Phase Angle - The angle between the Earth and Sun as seen from the body.
```

## Sub- and Superscripts

If some of the numbers or units on the fact sheets look a little strange, it may be that the browser you are using doesn't support sub- and superscripts. Such browsers include versions of Netscape and Mosaic below 2.0. On your browser, the number formatted to look like 10 to the minus 15 power (10 followed by a superscripted -15) looks like 10-15. If "10-15" looks like "10-15" to you, you will have trouble reading parts of the factsheets. In the explanation of units, we've included a description of the units in the form where, for example, 10^24 equals 10 to the 24 power.

Author/Curator: Dr. David R. Williams, dwilliam@nssdc.gsfc.nasa.gov, NSSDC, Mail Code 633 NASA Goddard Space Flight Center Greenbelt, MD 20771 301-286-1258
###### NASA Official: J. H. King, joseph.h.king@gsfc.nasa.gov Last Updated: 01 September 1999, DRW

Last Update: 2004-Nov-27