The second issue is that ruby does not have a fast arbitrary precision number class.
A float does not have high enough precision to store the Astronomical Julian Date, as it isn't able to get nanosecond level resolution for times in any recent date, and dates far in the future would would have less precision.
BigDecimal would be a possibility, and it is actually about 4 times faster than Rational even for simple fractions on 1.8.
However, on 1.9, Rational is implemented in C and is about two and a half times faster than BigDecimal.