Julian day and Day of Year (DOY) are NOT the same thing

I recently wrote a paper looking at how frog breeding timing is impacted by climate change. So, I’ve been reading lots of ecological studies of phenology (more on phenology later). One thing that struck me is how almost everyone in ecology misuses the term “Julian Day” when they mean Day-of-Year.

Day-of-Year (DOY), as the name suggests, is the count number of a given day in the year. So, Jan 25 is DOY 25 and March 1 is either DOY 60 or DOY 61 depending if it is a leap year. And we can express the time of day as a decimal, so that 3pm on January 1 is DOY 1.625.

Julian day is a completely different way to measure time. It was defined by an astronomer named Joseph Scalinger back in 1583 (and so, takes serious precedent over contemporary ecologists trying to hijack the term).

The point is, DOY and Julian day/date are wildly different things designed to measure wildly different phenomena.

Unlike DOY that starts counting on January 1^st in any given year, the Julian day count starts on January 1, 4713 BC. There is a complicated historical reason that Scalinger chose 4713 as the starting date that had to do with wedding the Julian and Gregorian dates during the calendar reform (read all about that here), but the point is, DOY and Julian day/date are wildly different things designed to measure wildly different phenomena.

For instance, I’m writing this blog on the 25^th of January 2020.

The DOY today is: 25

The Julian day today is: 2458873

But, it gets even crazier because unlike the DOY count that starts at midnight, Julian days start counting at Noon. So, right now, at 1030am the Julian day is 2458873, but after lunch it will be 2458874.

The Julian day metric is essentially worthless for comparing seasons. There is no ecologist who uses true Julian days; so, please, ecologist, don’t say Julian Day when you mean Day-of-Year.

As Gernot Winkler, former USNO Timer Service director notes:

“[Mixing Julian Day and DOY] is a grossly misleading practice that was introduced by some who were simply ignorant and too careless to learn the proper terminology. It creates a confusion which should not be taken lightly. Moreover, a continuation of the use of expressions “Julian” or “J” day in the sense of a Gregorian Date will make matters even worse. It will inevitably lead to dangerous mistakes, increased confusion, and it will eventually destroy whatever standard practices exist.”

So why does everyone misuse Julian Day? My hunch is that Julian Day sounds more technical than DOY, so folks gravitate toward it and others follow suit without ever questioning what it means.

Why do we care about studying seasonal change across years?

Phenology is the study of seasonal cycles of lifehistory like when bears go into hibernation, when flowers open, or when geese migrate. Phenology is a hot topic these days because climate change is causing wild populations to change their seasonal timing (Thackeray et al. 2016). For instance, frogs increasingly start calling and breeding earlier (Li et al. 2013) and forests green-up earlier (Cleland et al. 2007).

On one hand, shifts in lifehistory timing might be a good way to cope with climate change, but it can be bad news if shifts in one species causes a misalignment in an ecological relationship (Miller-Rushing et al. 2010; Visser & Gienapp 2019). For example, European flycatcher migration generally coincides with a boom in caterpillars that feed on oaks. However, climate change drives oaks to bud earlier, which means that all the juicy caterpillars turn chrysalises before the birds show up (Both & Visser 2001; Both et al. 2006). Similarly, snowshoe hares evolved to change coat color from white to brown in winter, but as snow melts earlier and earlier each year, rabbits are stuck with white coats for too long and become easy targets for predators (Mills et al. 2018).

Needless to say, it is important for use to be able to compare when in the season these critical phenomena take place and compare their change across years. When we do so, we are using DOY to align datasets across year, not Julian day; so, ecologists, let’s stop using the wrong term.

---

References:

Both, C., Bouwhuis, S., Lessells, C. M., and Visser, M. E. (2006). Climate change and population declines in a long-distance migratory bird. Nature 441, 81–83. 

Both, C., and Visser, M. E. (2001). Adjustment to climate change is constrained by arrival date in a long-distance migrant bird. Nature 411, 296–298. 

Cleland, E. E., Chuine, I., Menzel, A., Mooney, H. A., and Schwartz, M. D. (2007). Shifting plant phenology in response to global change. Trends Ecol. Evol. 22, 357–365. 

Li, Y., Cohen, J. M., and Rohr, J. R. (2013). Review and synthesis of the effects of climate change on amphibians. Integr. Zool. 8, 145–161. 

Miller-Rushing, A. J., Høye, T. T., Inouye, D. W., and Post, E. (2010). The effects of phenological mismatches on demography. Philos. Trans. R. Soc. Lond. B Biol. Sci. 365, 3177–3186. 

Mills, L. S., Bragina, E. V., Kumar, A. V., Zimova, M., Lafferty, D. J. R., Feltner, J., et al. (2018). Winter color polymorphisms identify global hot spots for evolutionary rescue from climate change. Science 359, 1033–1036. 

Thackeray, S. J., Henrys, P. A., Hemming, D., Bell, J. R., Botham, M. S., Burthe, S., et al. (2016). Phenological sensitivity to climate across taxa and trophic levels. Nature 535, 241–245. 

Visser, M. E., and Gienapp, P. (2019). Evolutionary and demographic consequences of phenological mismatches. Nat Ecol Evol 3, 879–885. 

The featured image of this post is from joiseyshowaa under creative commons usage.