approximately five years. Irregularities in the otherwise very smooth structure of the shells are conjectured to result from interactions between the winds of the two central stars, and from their orbital motion. Analysis of the expansion of the nebula has given a time for its formation at . This date is inconsistent with the peaks in brightness and with estimates of the periastron passage of the secondary star. Distance The Homunculus and Eta Carinae are assumed to be at approximately the same distance as Trumpler 16 and the Carina Nebula, but the distances to these objects are not known with any great accuracy. Instead, the distance of the Homunculus Nebula itself can be calculated using measurements of its expansion. The velocity of particular locations within the thin shell of the Homunculus lobes can be measured using the doppler shift of the spectral lines at that point, assuming the lobes are symmetrical. There are two different doppler shifts visible in the spectrum: one for direct emission lines; and one for reflected lines from Eta Carinae A. The direct line doppler shift gives the expansion velocity of the shell project onto the line of sight. Assuming an origin in Eta Carinae's Great Eruption and a constant expansion velocity, this gives the linear distance of the shell from the central star projected along the line of sight. The difference between the reflected line doppler shift velocity and the direct line velocity gives the distance of the shell from the central star, again assuming expansion at constant velocity since the Great Eruption. Observations of the spectrum of the Homunculus at a particular angular distance from the central star has shown the actual linear distance of that point from the central star, which defines the distance. Values obtained using this method are around with a margin of error around . The same calculations also return the inclination of the axis of the Homunculus relative to the line of sight. This turns out to be 41°, or 49° relative to the plane of the sky, which means it is slightly more "end on" than "side on". References External links 3D Homunculus Nebula Astronomy Picture of the Day 2014 July 17 Expanding Homunculus Nebula Astronomy Picture of the day (animated GIF) Little Homunculus and Butterfly Nebula Carina Nebula Emission nebulae Carina (constellation) Reflection nebulae