<p>Planets and moons reflect sunlight, as <a href="https://astronomy.stackexchange.com/questions/23232/planets-visible-at-night#comment39889_23232">mentioned in this comment by AtmosphericPrisonEscape</a>.</p>

<p>Now I'll add a bit more info to make this post more informative :)</p>

<p>That also means that just like the moon, <a href="https://en.wikipedia.org/wiki/Planetary_phase" rel="nofollow noreferrer">the planets have phases</a>, but they're just too subtle to see with the naked eye, and for any planet further from the Sun than the Earth is, we don't see all the phases.  They're full when the planet is furthest from us (on the other side of the Sun), so it'd be hard to tell the phase just from how bright the planet appears.</p>

<p>But through a telescope, you can easily see the phases of Venus, and you can tell it's smaller when it's full (because it's farther away):</p>

<p><a href="https://i.sstatic.net/UJ1w2.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/UJ1w2.jpg" alt="Phases of Venus"></a><br>
<em>Source: <a href="https://commons.wikimedia.org/wiki/File:Phases_Venus.jpg" rel="nofollow noreferrer">Wikimedia Commons</a></em></p>

CC BY-SA 3.0
2017-10-23T23:31:44.877Z
Great answer! I've asked the follow-up question [How (the heck) was this photo of Venus at inferior conjunction (between us and the Sun) taken?](https://astronomy.stackexchange.com/q/23271/7982)
uhoh
CC BY-SA 3.0
2017-10-20T10:01:56.507Z
2017-10-20T10:27:26.457Z
2017-10-20T10:27:26.457Z
gerrit
<p>This is incorrect.  All bodies give off light according to Planck's Law.  The amount of emitted light may be small compared with reflected light, or small compared to the sensitivity of your detector.  </p>

<p>This may seem like nitpicking, but failure to incorporate emittance when analyzing spectra will lead to sadness. </p>

CC BY-SA 3.0
2017-10-20T15:12:13.750Z
Technically true. But most planets have their emission peaks in the infrared, so that wouldn't amount to much to the human eye, which is a photometer and not a spectrometer.
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-10-20T19:34:39.270Z
Yes, but in the context of the original question, this is so "technically true" as to be misleading.
Florin Andrei
CC BY-SA 3.0
2017-10-20T13:44:25.267Z
2017-10-20T13:44:25.267Z
Carl Witthoft
<p>Stars give off light and are visible at nighttime but Planets do not give off light . So why are Planets visible at nighttime ?</p>

CC BY-SA 3.0
2017-10-19T22:42:46.623Z
They reflect sunlight, just like the moon.
AtmosphericPrisonEscape
<planet><night-sky>
Planets visible at night
Peter U
# Planets visible at night

(Asked by: username_0 on 2017-10-19)

Stars give off light and are visible at nighttime but Planets do not give off light . So why are Planets visible at nighttime ?

**Tags:** planet, night-sky
> **Comments:**
> **username_1**: They reflect sunlight, just like the moon.  

---

## 2 answers

### Answer by username_2 on 2017-10-20. Score: 4

Planets and moons reflect sunlight, as [mentioned in this comment by AtmosphericPrisonEscape](https://astronomy.stackexchange.com/questions/23232/planets-visible-at-night#comment39889_23232).

Now I'll add a bit more info to make this post more informative :)

That also means that just like the moon, [the planets have phases](https://en.wikipedia.org/wiki/Planetary_phase), but they're just too subtle to see with the naked eye, and for any planet further from the Sun than the Earth is, we don't see all the phases. They're full when the planet is furthest from us (on the other side of the Sun), so it'd be hard to tell the phase just from how bright the planet appears.

But through a telescope, you can easily see the phases of Venus, and you can tell it's smaller when it's full (because it's farther away):

[![Phases of Venus](https://i.sstatic.net/UJ1w2.jpg)](https://i.sstatic.net/UJ1w2.jpg)  
*Source: [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Phases_Venus.jpg)*

> **Comments:**
> **username_3**: Great answer! I've asked the follow-up question [How (the heck) was this photo of Venus at inferior conjunction (between us and the Sun) taken?](https://astronomy.stackexchange.com/q/23271/7982)  

---

### Answer by username_4 on 2017-10-20. Score: -1

This is incorrect. All bodies give off light according to Planck's Law. The amount of emitted light may be small compared with reflected light, or small compared to the sensitivity of your detector.

This may seem like nitpicking, but failure to incorporate emittance when analyzing spectra will lead to sadness.

> **Comments:**
> **username_1**: Technically true. But most planets have their emission peaks in the infrared, so that wouldn't amount to much to the human eye, which is a photometer and not a spectrometer.  
> **username_5**: Yes, but in the context of the original question, this is so "technically true" as to be misleading.  

---

