+---+------+-----+----------------------------------------------+ | | | 1 | Light after passing through atmospheres of | Line| Wave-| - | the following thicknesses. | | |length| λ⁻⁴ +-+----+----+----+----+----+----+----+----+----+ | | |×10¹⁷|0| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 32 | +---+------+-----+-+----+----+----+----+----+----+----+----+----+ | A | 7594 | 30 |1|·955|·908|·857|·815|·775|·736|·707|·665|·107| | B | 6867 | 45 |1|·926|·858|·795|·735|·684|·632|·583|·542|·086| | C | 6562 | 54 |1|·912|·832|·759|·693|·632|·576|·526|·480|·019| | D | 5892 | 83 |1|·868|·754|·655|·569|·494|·428|·372|·323|·001| | E | 5269 | 129 |1|·803|·644|·518|·427|·334|·268|·216|·173| -- | | F | 4861 | 179 |1|·738|·544|·402|·296|·219|·161|·119|·088| -- | | G | 4307 | 291 |1|·609|·367|·220|·137|·084|·051|·031|·019| -- | | H | 3968 | 403 |1|·506|·254|·128|·071|·033|·016|·008|·004| -- | +---+------+------+-+----+----+----+----+----+----+----+----+----+ The sun traverses the following thicknesses of atmosphere when it is at the angles shown above the horizon. 1 atmosphere 90° 2 " 30° 3 " 19·30 4 " 14·30 5 " 11·30 6 " 9·30 7 " 8·30 8 " 7·30 Fig. 10.--Absorption of Rays by the Atmosphere. It traverses thirty-two atmospheres when it is very nearly setting. Bougier and Forbes have calculated that the extreme thickness of the atmosphere, traversed by its light when the sun is on the horizon, is approximately 35-1/2 atmospheres. The absorption shown by 32 atmospheres will therefore be very close to that which would be observed at sunset on an ordinary day, and it will be seen that practically all rays have been scattered from the light, except the red, and a little bit of the orange. As to the luminosity of the sun at these different altitudes, we can easily find it by reducing the luminosity curve of the sun at some known altitude by the factors in the table just given, for as many wave-lengths as we please, and thus construct another curve. The area of the figure thus obtained would be a measure of the total luminosity on the same scale as the area of the luminosity curve from which it was derived. The following are the approximate luminosities of the sun when the light shines through 0 atmospheres 1 " 1 " ·840 " 2 " ·705 " 3 " ·594 " 4 " ·496 " 5 " ·417 " 6 " ·303 " 7 " ·256 " 8 " ·215 " 32 " ·002