ON SOLAE PHYSICS. 
13 
1. Year, month, and day. 
2. Running number. 
3. Mean time of sun-picture. 
4. Number of group in the Kew Catalogue. 
5. Distance from centre. 
6. Angle of position. 
7. Heliographical longitude from node. 
8. True heliographical longitude. 
9. Heliographical latitude. 
10. Letter denoting the particular spot of the group. 
The “ heliographical longitude ” is derived from the “ longitude from node ” in the 
following manner. Taking that great circle through the sun’s poles, which at the epoch 
1854'0 passed through the ascending node as first meridian, and assuming preliminarily 
25 '38 days as the sun’s period of rotation, this prime meridian will obviously again 
coincide with the node at the periods 
1854-0+25 d -38, 1854*0+2(25 d -38), 1854+rc(25 d -38). 
A spot being on the sun at the time 1854*0, the calculated longitude of which was 
found to be 0 o- 0, would be on the prime meridian ; but the calculated longitude of the 
same spot, a day afterwards, would be found to be - = 14° ll f 3"*84 (disregarding for 
our present purpose all modifying influences), this being its “ longitude from node.” 
To reduce this longitude to the true heliographical, a small table is necessary, showing 
at a glance the civil dates of the times 1854*0-]-25 d ’38+25 d, 38 + 
For a sun-picture taken intermediately between two coincidences of the prime meri- 
dian with the nodal point, the time since the preceding coincidence is found by sub- 
traction ; and calling this time T, we have obviously 
T . 360° 
longitude of prime meridian from node = 25 , 38 = 1! ; 
and if the longitude from node of a spot be=Z, its angular distance from the prime 
meridian, reckoning always from west to east, will be 
l — =the “ Heliographical Longitude.” 
