288 Mr. R. P. Greg on Luminosity of Meteors 
Sir J. Lubbock and others. The very sudden appearance and 
disappearance of shooting-stars and small meteors, and their 
general resemblance on a small scale to comets which shine 
by solar reflexion, certainly favour the idea, either that suddenly 
entering the cone of the earth’s shadow they are instantly 
eclipsed, or conversely, become visible as they emerge from it ; 
or secondly, previously self-lumimous in planetary space, they 
may become suddenly extinguished on entering the denser atmo- 
sphere of the earth; or thirdly, they may suddenly become 
visible and luminous, only on entering the earth’s atmosphere by 
friction and compression, by rapid absorption of oxygen and 
sudden chemical action, or by electrical excitation. 
I shall consider the first supposition most fully and in the 
first instance, because I consider it may be most readily and 
completely disproved. Sir J. Lubbock, in an interesting paper 
in this Magazine for February 1848, and one that has since 
been frequently referred to, considers the hypothesis of solar re- 
flexion as a very applicable one in certain varieties of shooting- 
stars: he even says, “knowing the time when, and the place 
where the star disappeared, the elements of the geometry of three 
dimensions furnish the means of determining the exact distance 
of the body from the place of the spectator or from the centre 
of the earth;” and in his paper he gives several geometrical 
equations and formule for assisting such determinations. I do 
not propose entering into the nature of these calculations, or to 
question either the results or the data, but merely by a different 
treatment to show, if I can, how unlikely, if not impossible, it is 
that ordinary shooting-stars (I mean, of course, those not show- 
ing symptoms of active ignition within the lower limits of the 
earth’s atmosphere) can ever shine by reflected solar light ; and 
this simply from the fact that they would be quite too far off for 
us to observe such small bodies, at even the minimum distance 
at which (at certain times and places on the earth’s surface when 
and where we know they are very frequently seen) they actually 
could be so visible. 
The problem I propose then to solve is, what is the minimum 
distance at which a shooting-star could be thus visibly luminous 
seen at an angle, say of 45 degrees above the horizon (the ma- 
jority of shooting-stars appearing, as a rule, to the spectator 
at even a greater angle), to an observer situate at midnight 
within the tropics, or, to be more precise, at the equator, about 
the time of the vernal or autumnal equinox. 
In fig. 1, let S be the centre of the sun, and E that of the 
earth, and 8 A and EC semi-diameters of sun and earth respect- 
ively ; let SE=95,000,000 miles, BS and CE=8950 miles, 
and S A=422,500 miles; then supposing the shadow of the 
