It has to be visible 360 degrees for two miles.
Sort of. See Colregs Annex I 84.17
(b) All-round lights shall be so located as not to be obscured by masts,
topmasts or structures within angular sectors of more than 6 degrees,
except anchor lights prescribed in Rule 30, which need not be placed at
an impracticable height above the hull, and the all-round white light
described in Rule 23(d) [
not an anchoring rule] , which may not be obscured at all.
Also:
2. Vertical positioning and spacing of lights
(a) On a power-driven vessel of 20 meters or more in length the masthead
lights shall be placed as follows:
(i) the forward masthead light, or if only one masthead light is carried,
then that light, at a height above the hull of not less than 6 meters,
and, if the breadth of the vessel exceeds 6 meters, then at a height
above the hull not less than such breadth, so however that the light
need not be placed at a greater height above the hull than 12
meters;
(ii) when two masthead lights are carried the after one shall be at least
4.5 meters vertically higher than the forward one.
(b) The vertical separation of masthead lights of power-driven vessels shall
be such that in all normal conditions of trim the after light will be seen
over and separate from the forward light at a distance of 1000 meters
from the stem when viewed from sea level.
(c) The masthead light of a power-driven vessel of 12 meters but less than
20 meters in length shall be placed at a height above the gunwale of not
less than 2.5 meters.
0. Vertical sectors
(a) The vertical sectors of electric lights as fitted, with the exception of lights
on sailing vessels underway
[anchored is not under way] shall ensure that:
(i) at least the required minimum intensity is maintained at all angles
from 5 degrees above to 5 degrees below the horizontal;
(ii) at least 60 percent of the required minimum intensity is maintained
from 7.5 degrees above to 7.5 degrees below the horizontal.
(b) In the case of sailing vessels underway the vertical sectors of electric
lights as fitted shall ensure that:
(i) at least the required minimum intensity is maintained at all angles
from 5 degrees above to 5 degrees below the horizontal;
(ii) at least 50 percent of the required minimum intensity is maintained
from 25 degrees above to 25 degrees below the horizontal.
(c) In the case of lights other than electric these specifications shall be met
as closely as possible
Note that this explains why anchor lights can be nearly invisable within a few hundred feet; you are under the required angle. You need to view your anchor light from at least 500 feet to get an accurate idea of brightness. And this is why suplimental lights are smart.