Which side of Mars in the telescope is facing us today?
First understand Mars' own north and south, longitude and light direction, and then understand the disk in the photo.
Planets and Small Solar System Objects
Mars doesn't always point the same hemisphere and the same longitude towards us. It rotates, and the Earth is oriented differently from the Sun; these factors together determine the disk's tilt, central meridian, and brightness.
Guess first
Does the hemisphere of Mars facing the Earth also face the Sun?
Show answer and explanation
uncertain — The Earth and Sun are usually not in the same direction as seen from Mars.
Then connect experience and science
First, distinguish between the North Pole of Mars and the Northern Heavens.
P gives the orientation of Mars' north pole axis in the sky, which is not equal to your latitude.
Mars polar axis direction → observation position angle P
The hemisphere seen by the Earth is different from the hemisphere illuminated by the Sun
DE describes the direction of the earth and DS describes the direction of the sun. These two values can be one positive and one negative.
DE and DS are calculated separately
Mars rotates and its central meridian changes
ω tells you which meridian is in the center of the disk at this moment; rotation requires the use of luminous time, and cannot ignore the time it takes for light to reach the earth.
W uses JD − light travel time
Calculate light, dark and size simultaneously
The phase determines the bright area and deficiency, and the distance from the center of the earth determines the apparent diameter; the size of the illustration does not correspond to the actual angular resolution of the mobile phone screen.
d=9.36″/Δ；q=(1−k)d
First find the directions of the earth and the sun as seen from Mars, and obtain DE and DS respectively; W uses the time of subtracting the light travel time. d is the angular diameter, q is the angular width of the unilluminated part, both are expressed in arcseconds.
give it a try
Keeping the marked longitude unchanged, push it for a few hours; then compare the disk orientation with the direction of the sun. The markings are only teaching marks, not the actual Martian landscape.
No need to memorize symbols
- DE / DS
- Declination of the Earth/Sun as viewed from Mars
- ω / P
- Central meridian longitude/North Pole axis position angle
- χ / k / q / d
- Bright edge position angle/bright area ratio/maximum defect/apparent diameter
Algorithm, accuracy and scope of application
Realize the complete main calculation chain of Chapter 42: the polar ecliptic direction starting from J2000, the position of the earth at a fixed observation time, the position of Mars after repeatedly calculating the light travel time, DE/DS, the rotation angle W and the central meridian ω, the optical aberration and the axis position angle P after nutation, and brightness Edge position angle χ, apparent diameter 9.36″/Δ, illumination ratio and deficiency. Rely on Chapter 32/33 same-day equinox truncation VSOP87 and Chapter 25 nutation/yellow-red intersection angle, do not call the first page approximation. Original book 42.a uses 1992-11-09 UT is added to the original ΔT for about 59 seconds; the input is interpreted as TD. The image surface markers and colors are for illustration and do not simulate real Mars maps, polar caps, dust storms or local station center atmospheric effects; they cannot replace precision observation software.
my understanding check
What is the first thing to do with the changes in the central meridian?
Mars rotation and observational geometry — It is also necessary to use the corrected state of Mars using light travel time.
Is P the position of Mars' true North Pole in Earth's latitude?
No, it’s the direction angle of the central axis in the sky — It describes image orientation, not ground position.
Are the small marks in the picture the latest topographic map of Mars?
no — It only helps to understand how a fixed longitude moves in and out of view as it rotates.
The Martian disk requires three sets of information: which meridian it is turning on, which way the Earth is looking from, and which way the Sun is shining.
Jean Meeus · Astronomical Algorithms, 2nd ed. · No. 42 chapter
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