28 
POPULAR SCIENCE REYIEW. 
greatest than to his least limit of distance. Thus, whereas the 
greatest diameter Mars can present is no less than 24", and his 
least only 13‘3", his apparent diameter on January 10 will be 
only 14-0". There is an error, by the way, in the Nautical Al- 
manac on this point ; and not a single error, due to an accident 
or misprint, but an error running through the whole series 
of apparent magnitudes for several months. In fact, a larger 
diameter is attributed to the planet than at the opposition of 
1864, an error which a moment’s inspection of fig. 1 is sufficient 
to correct.* 
Next let us consider what is the polar presentation of Mars 
at the approaching opposition. Eeferring to fig. 1, we see that, 
since Mars was at the vernal equinox of his northern hemi- 
sphere when at M 1? about 2J- months ago, he will be somewhat 
more than one-third of the way towards the northern summer 
solstice at the time of opposition. Thus the northern pole will 
be turned towards the sun, and therefore towards the earth ; but 
somewhat less towards the earth than towards the sun on account 
of the elevation of Mars above the ecliptic. Again, when at M 1? 
Mars, viewed from the sun, would have appeared with his axis 
apparently inclined nearly 30° to the direction of his motion ; 
at the time of observation this angle will have considerably di- 
minished. The actual presentation of Mars will be very nearly 
that exhibited in the eight pictures of our illustrative plate.f 
In these figures, details observed by Dawes in 1864, by Lockyer 
and Phillips in 1862, and by Delarue in 1858, have been intro- 
duced so as to accord with the actual presentation of 1867. I 
have not found it an easy matter satisfactorily to reconcile these 
designs ; neither, indeed, is it to be assumed that Mars presents 
at all seasons identical features. It is found, in fact, that, besides 
periodic changes in the dimensions of those two white caps near 
the polar regions which have so long been recognised as 
The snowy poles of moonless Mars, 
the details of other portions of his surface vary from time to 
time. Spots and patches clearly made out on one occasion 
appear blurred and indistinct on another — though the same 
* I should have felt in doubt as to the correctness of my own calculations, 
"but for (i.) the extreme simplicity of the question, and (ii.) the circumstance 
that the results given in the Nautical Almanac are self-contradictory ; the 
horizontal parallax (always proportional to the apparent magnitude) and 
the distance of the planet being correctly given — the first less, the second 
greater, than the corresponding elements in 1864. 
t In these figures the horizontal line through the centre indicates the 
direction of the planet’s motion. His apparent motion across the field of the 
telescope will take place in a direction inclined a few degrees to this hori- 
zontal line, crossing it from below upwards, and from right to left. 
