heyoka.model.dayfrac

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heyoka.model.dayfrac#

heyoka.model.dayfrac(time_expr: expression = heyoka.time) → expression#

Number of fractional days elapsed since January 1st.

Added in version 7.4.0.

The input time expression time_expr is assumed to represent the number of fractional terrestrial time (TT) days elapsed since the J2000 epoch.

The returned value is the number of fractional TT days elapsed since January 1st, 00:00 UTC, of the calendar year corresponding to time_expr. A fractional day is expressed as a decimal count of 24-hour days (e.g., 1.25 represents 1 day and 6 hours).

TT days are always exactly 86400 SI seconds long, so in years with leap seconds, the returned quantity may differ slightly from the UTC day-of-year (DOY) due to the differing length of UTC days (86399 or 86401 SI seconds). This quantity is therefore similar to DOY, but:

  • it is zero-based (Jan 1 00:00 corresponds to 0.0 rather than 1.0),

  • it is measured in TT days, not UTC days.

Computation is always internally performed in double precision, even when evaluating expressions at higher numerical precision. This design choice reflects its intended use in applications such as thermospheric models, where the model uncertainty far exceeds any numerical error from double precision or small offsets from UTC DOY.

Parameters:

time_expr – the number of TT days elapsed since the J2000 epoch.

Returns:

the number of TT days elapsed since January 1st, 00:00 UTC, of the calendar year corresponding to time_expr.

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