ESA Sentinel-1A Image: Lake County, Oregon from orbit

Credit: ESA

This image from Sentinel-1A was acquired over the Lake County in the US state of Oregon on 17 July.

Even though the northwestern United States receives plenty of rainfall, the 'high desert region' east of the Cascade Mountain Range is fairly dry.

Most crops require irrigation, such as the circles visible in this image from a centre pivot irrigation system.

Sentinel-1A's radar gathers information in either horizontal or vertical radar pulses, and colours were assigned to the different types.

Sentinel-1A satellite

In this image, rough surfaces, like vegetated areas, appear white and light blue while smoother surfaces are red and black.

Some of the black areas are, or once were, lakes. These lakes formed when glaciers melted after the last ice ages, but dried up as the climate became drier.

Running north–south through the centre of the image we see a line of evenly dispersed red dots.

These are reflections from metal towers holding power lines. While these structures are difficult to spot in optical imagery, their reflectivity makes them more visible to radar.

Launched in April, Sentinel-1A recently completed its commissioning phase, ensuring that the satellite, instruments, data acquisition and data processing procedures are working well.

During this phase, it manoeuvred eight times to avoid space debris.

The satellite is now fully operational delivering radar data for an array of services and scientific research.

Rare Moonbows or Lunar Rainbows over Oregon and Hawaii

We have all seen rainbows during the day but what about rainbows at night? Thomas Thies of Gearhart, Oregon photographed this one after the sun had set on December 8th 2003.

He says, "The rainbow, which lasted for about 20 minutes, was out over the ocean with the moon rising behind us." The several bright yellow lights near the horizon are crab boats.

The moon is the key, just as sunlight produces rainbows during the day, moonlight can produce rainbows at night. This is a lunar rainbow or 'moonbow'.

Moonbows are rare because moonlight is not very bright. A bright moon near to full is needed, it must be raining opposite the moon, the sky must be dark and the moon must be less than 42º high.

Put all these together and you do not get to see a moonbow very often! To the unaided eye they usually appear, as in the small image, without colour because their light is not bright enough to activate the cone colour receptors in our eyes. Nonetheless colours have been reported and might be seen when the moon is bright.

Moonbow and Venus imaged by Rob Ratkowski (photography) in May 2004. Moonbows are very rare.

The moon must be bright and not too high, the sky must be dark else the faint bow will not be visible and there must be rain in the direction opposite the moon.

Orion was setting when Rob took this spectacular image, Venus is the bright object inside the bow.

Mars is above it to the left and Saturn is above Mars and outside the primary. Capella is the bright star to the right (north) of Venus.

Do not expect to see much colour with the unaided eye because moonbows are usually too faint to excite the colour sensors in our eyes. We therefore mostly see them as a series of grey shades.