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I know that Venus is visible during sunset and sunrise, but is it also visible in deep dark night?

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James K has provided a short answer "no". The longer answer is that because Mercury and Venus are in closer orbits to the Sun than Earth's, they will always be in roughly the same area of the sky as the Sun, whereas the other planets aren't so restricted. Since something is generally only visible if it's above the horizon, it follows that if the Sun is below the horizon, Mercury and Venus will either be close to or below the horizon.

The very furthest that Venus gets from the Sun (its "elongation") is 47.8°. If Venus is at its peak elongation around the time of the summer solstice, its angle above the horizon around sunrise (or sunset, depending on which side of the Sun Venus is on at the time) will be maximised. Add to this that since Venus is very bright at this elongation, it's actually visible even when the Sun is low above the horizon; and it remains visible for about three hours after sunset (or, if it's on the Sun's other side, it rises above the horizon about three hours before sunrise). In winter, on the other hand, the ecliptic is substantially lower and Venus's elevation at sunrise/sunset is therefore much less.

If the Sun is more than 47.8° below the horizon, then Venus must also be below the horizon. You can never see Mercury or Venus in "deep dark night" (e.g. around midnight), whereas Mars, Jupiter and Saturn are readily visible (and quite bright!) if they happen to be above the horizon at midnight. [NB I'm excluding the situation of summer at extreme latitudes, since that isn't "deep dark night"].

This diagram on Wikipedia provides a simple representation of elongation and angular separation, and shows the difference in how the "inner" and "outer" planets can appear in the sky.

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The short answer is no, but it might be visible for longer than you expect.

When Mercury or Venus is furthest from the sun they are said to be at "Elongation". During favourable elongation that occurs at the end of 2018, Venus rises (in my location) at about 4am, four hours before the sun, and Venus will be in the sky when most people would say it is "dark"

The previous Evening elongation in August 2018 is less favourable from the Northern Hemisphere, but from the southern hemisphere Venus won't set until the sky is dark.

This contrasts with Mercury, which is only visible in the twilight as it is much closer to the sun. The other planets can be see at midnight when they are near opposition.

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