  The HST archive contains a large collection of images of eta Carinae, and
this paper analyzes those most suitable for measuring its expanding Homunculus
Nebula. Multiple intensity tracings through the Homunculus reveal the
fractional increase in the overall size of the nebula; this avoids registration
uncertainty, mitigates brightness fluctuations, and is independent of previous
methods. Combining a 13-yr baseline of Wide Field Planetary Camera 2 (WFPC2)
images in the F631N filter, with a 4-yr baseline of Advanced Camera for
Surveys/High Resolution Channel (ACS/HRC) images in the F550M filter, yields an
ejection date (assuming linear motion) of 1847.1 ($\pm$0.8 yr). This result
improves the precision, but is in excellent agreement with the previous study
by Morse et al.\ (2001) that used a shorter time baseline and a different
analysis method. This more precise date is inconsistent with ejection during a
periastron passage of the eccentric binary. Ejection occured well into the main
plateau of the Great Eruption, and not during the brief peaks in 1843 and 1838.
The age uncertainty is dominated by a real spread in ages of various knots, and
by some irregular brightness fluctuations. Several knots appear to have been
ejected decades before or after the mean date, implying a complicated history
of mass-loss episodes outside the main bright phase of the eruption. The
extended history of mass ejection may have been largely erased by the passage
of a shock through clumpy ejecta, as most material was swept into a thin shell
with nearly uniform apparent age.
