By Duncan Steel 19/02/2019

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It’s easy to see the International Space Station passing overhead: you just need to know when and where to look. Oh, and a clear sky.

The International Space Station (ISS) regularly passes across New Zealand, a little more than 400 km above our heads – rather less than the distance between Auckland and Wellington. Most of these transits occur either in daytime, making the ISS difficult (though not impossible) to see against the sky, or in the depths of night, when it is dark because it is in Earth’s shadow. Every so often, however, the geometry is right and the ISS can be seen in the dark sky either in the evening or the morning, after sunset or before sunrise; whilst down below we are in darkness, the ISS in orbit may still be illuminated by sunlight, making it an obvious bright spot of moving light.

People tend to ask “How easy is it to see?” I wrote above that it’s obvious, and that is indeed the case. In many of the viewing opportunities it is brighter than the brightest star in the sky, and its reflected sunlight can be as intense as that of Venus (often referred to as the Morning or Evening Star). What makes the ISS different to the most luminous stars, and also the brightest planets (Venus, Mars, Jupiter, Saturn), is that is moving, rather quickly. In its orbit its speed is almost 7.7 km per second (over 27,500 kph), and in consequence for a viewer on Earth’s surface below it moves from horizon to horizon in typically six minutes, depending on the precise angles.

The ISS, then, looks like a bright, fast-moving, high-altitude jetliner when you see it after dusk. There is, though, no mistaking it in that its appearances from different locations can be predicted with precision, knowing to the second when it will rise above the horizon, and when it will slip into Earth’s shadow.

Calculating such things is easy: you just need to access a website such as the wonderful Heavens-Above, where you can enter your location (either your city, or your latitude and longitude), define which satellite you’d like to get predictions for, and hit Go. Alternatively you can access this NASA website, and arrange for emails to be sent to you whenever the ISS will be visible from your location within the next 24 hours.

For the ISS, typically there will be an interval of about 7-10 days over which it can be seen from NZ repeatedly, but then there will be a several weeks across which it is not available: it is passing over our territory during the day and the depths of night, and not at the right times to be visible at dawn or dusk.

Well, it happens that we have just entered one of those happy times when the ISS can be witnessed crossing our sky, from now until the end of the month. The precise times and places to look in the sky vary across NZ, as you might expect given that it’s orbiting at an altitude of only about 408 km above the equator (though slightly more at our latitude).

In the tables below I present predictions through to 28th February (which is my younger son’s birthday). As someone whose surname has always put him near the bottom of any alphabetical queue, I thought I would arrange these ISS-spotting predictions in reverse order of latitude (i.e. starting at the south and then heading north), but then I noticed that the southern parts of NZ get far more opportunities to see the ISS crossing the evening sky, and better chances too in that it generally will be brighter for the far southern parts. So, there are distinct advantages in living in places like Alexandra.

In these tables (one for each of 14 cities/towns) the first two columns give the date and then the time (NZDT) at which the viewing opportunity begins. This is defined by the ISS being bright enough to be easily seen with the naked eye (clouds allowing) and also at least ten degrees above the horizon.

The Duration column indicates in minutes and seconds how long you can watch the ISS from that first appearance (crossing the ten-degree elevation angle) until it disappears; for these viewing opportunities, all in the evening sky, the disappearance time may be defined by the ISS dipping below ten degrees elevation angle again, or may be set by the time it will become dark because it passes into Earth’s shadow. Watching as the latter phenomenon occurs is interesting because the shadow edge is not sharp, but rather is delimited by the tenuous upper layers of the atmosphere, and so the ISS dims over a few seconds until one can no longer detect it.

The Arc of Directions column gives three directions in terms of compass points: where the ISS will first appear, the direction at which it reaches its highest elevation angle (or, minimum zenith angle) for that pass, and the compass direction in which it will disappear. This gives an indication of the path it will follow across the sky, and so where to look.

The Maximum Elevation is that highest angle above the horizon that the ISS will reach, for each pass/viewing location. The 90°-complement of that angle is the zenith angle: the angular distance between that highest-elevation point and the zenith (i.e. the point directly overhead). Large elevations/small zenith angles mean that it will pass close to overhead.

Lastly I have indicated with a black dot on the right of the relevant rows in each table which of the passes are ‘best’. By that I mean a qualitative combination of brightness, duration, and elevation angle. If you are only going to venture out to see one ISS pass, choose one with a black dot.

Not living in/visiting any of those 14 places? Just use that closest to where you are as an indication; or interpolate between the figures given; or go to Heavens-Above and insert your specific location to get precise values.

Looking at the tables you may notice that on many dates there are two passes visible on the same evening, separated by about 98 minutes. You might guess that this is how long it takes the ISS to orbit the Earth, but you would not be quite correct. The orbital period of the ISS is near 92.7 minutes, briefer than the 98 minutes mentioned between observable passes; the difference is due to the fact that during each ISS orbit the Earth is also spinning below, so the space station needs to travel by more than a complete loop of the planet so as to come back over the same territory again.

Obviously a visualisation of the International Space Station in its orbit, and these passes over New Zealand in the next ten days, would be of use and interest to many readers. This is a long post already, and it takes me a while to produce such graphics and movies, but I will get a follow-up post done in the next few days.

Did you see what I did there? Yes, Nelson is north of Wellington, in terms of latitude.

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