See satellites in broad daylight!!
Iridium flares can be so bright that they are actually visible in daylight, even when the sun is
well above the horizon. As keen astronomers know, the planet Venus (Mag. -4) can be seen
during the daytime, provided it is not too close to the sun and one knows exactly where to look.
A magnitude -8 Iridium flare, which is about 40 times brighter than Venus, is much
easier to see, even though it only lasts for no more than a couple of seconds.
Only the very brightest flares can be seen in the daytime, and so they don't occur as frequently
as nighttime flares. They are even more sensitive to your position, so it is
extremely important to determine your position as accurately as possible. To avoid
dissappointment, you should try to get your position to an accuracy of about 2 km, which
corresponds to an accuracy in latitude/longitude of approximately 0.02° or 1 arc minute.
For more information, please see the Iridium flare help
page or visit the
Visual Satellite Observer's Homepage
Interpreting the Predictions
The following is an example of the daytime flare predictions table;
||3.2 km (E)
||183° (S )
||2.9 km (W)
||1.3 km (E)
||184° (S )
||3.1 km (W)
We will now explain each of the table columns in turn.
- The date on which the flare occurs (in local time). Several flares can occur on the same day.
- The exact time when the flare reaches its maximum brightness (in local time).
- The estimated maximum intensity of the flare in the standard astronomical magnitude scale.
This is a logarithmic scale and it is important to note that lower
numbers represent brighter flares. The scale is defined such that a
magnitude difference of 5 represents an intensity difference of factor 100. Thus a magnitude
-7 flare is one hundred times more intense than a magnitude -2 flare. As a comparison, the brightest
star in the sky (Sirius) has magnitude -1, the planet Venus (the brightest star-like
object) can reach magnitude -5, and the full Moon shines at magnitude -12. Please remember that
these magnitude estimates are only approximate, and are affected by errors in the
Iridium satellite attitude and position errors of the observer's location. A 10km position
error on the ground can easily result in an estimated magnitude error of several magnitudes.
- Flare position - Elevation
- This is the angle above the horizon, in degrees, at which the flare reaches its maximum intensity.
- Flare position - Azimuth
- This is the angle measured clockwise around the horizon from true north, at which
the flare reaches its maximum intensity. Thus, an azimuth angle of 0°
represents north, 90° is east, 180° is south and 270° is west. To make sure
you are interpreting the angles correctly,
start facing the northern horizon, then rotate clockwise through the azimuth angle
about the local vertical axis. Finally, look up from the horizon through the
- Flare centre - Distance
- The flare centre corresponds to the point on the earth´s surface where the flare is
brightest, and NOT the position of the satellite itself.
The information is given for people who want to travel to the flare centre to get the
The distance columns
gives the distance from the observers location, and the approximate direction. Since the
satellites are in near-polar orbits, the track of the flare along the ground is nearly
north-south or south-north, and so the direction to the centre is always eastwards or westwards.
- Flare centre - Mag.
- The magnitude of the flare as it would be seen at the centre.
- Sun - Elev.
- The elevation of the sun above the horizon at the time of the flare.
- Sun - Azimuth
- The azimuth of the sun at the time of the flare.
- Sun - From flare
- The angular separation of the sun from the flare. If the flare is too close to the sun,
it will be more difficult or impossible to see.
- The final column gives the name of the satellite producing the flare. Clicking on the name will
take you to that satellite's information page.
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Chris Peat, Heavens-Above GmbH|
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