Nobody wants to be deceived, but it happens to every one of us at some point in our lives. What brings this on astrologically? Neptune, the planet of illusion, delusion, and yes, deception. Neptune can create a fog that sets in and obscures our vision so we're easily taken advantage of. Once the fog lifts, we realize what's happened.
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Transit Neptune Aspects & Deception
You want to look at transit Neptune making hard aspects - conjunctions, squares, oppositions - to any of your natal planets, but especially your personal planets (Sun, Moon, Mercury, Venus, Mars). This is when you're most vulnerable to deception and need to guard yourself better.
Not only look at the planets in play, but also keep in mind the houses they're in and rule in your natal chart. Being swindled out of your money, for example, would show up with a hard aspect to the 2nd house cusp or ruler, the house of money. I always caution people about getting seriously involved with someone if they have a hard aspect by Neptune to their Venus, planetary ruler of love, 5th house cusp or ruler (house of love), or 7th house cusp or ruler (house of commitment).
Also track the movement by Neptune through your houses. What's ruled by those houses is where you're more generally easily deceived at the moment. This is a challenging one because it lasts much longer than a Neptune aspect to a natal planet (a year or two for the aspect to a natal planet, up to a decade for in a natal house). You can adjust a bit better with it in the houses over time, but may be more vulnerable in the first couple of years it's in effect.
You can look at what Neptune is doing to your progressed chart as well. If transit Neptune is doing any of the same things to your progressed chart (hard aspects to the progressed personal planets, changing progressed houses), this can also make you more vulnerable.
So how do you counter Neptune?
The problem with Neptune is, when we're under its influence with a hard aspect, it's so difficult to convince us that we're not being realistic. We swear we're seeing things exactly as they are, but we're not at all. That's the Neptune influence making us see what we want to see instead of what's actually there. Not only are we easily deceived by others, but we can easily deceive ourselves as well.
What's the exact opposite of this? Saturn influence, the eternal realist. Often, I find people have the hardest time with the consequences of difficult Neptune if they come out of the aspect with a hard Saturn aspect. It can cause a big crash to reality.
So whenever you see hard Neptune aspects coming, check yourself often, and listen when people you trust question your judgment. They may be seeing things that you just don't want to.
Suggested Reading: Transit Neptune and the Personal Planets